On Tuesday, United Airlines officially opened a huge new $500-million terminal at Chicago's busy O'Hare International Airport, trumpeting it as the "Terminal for Tomorrow." How farsighted its planners actually have been remains to be seen, of course, but the innovations that were unveiled are a good indication of what travelers can expect to find in remodeled airports elsewhere in the country in the next few years.

One of United's major goals is to ease the big problem of delayed flights at O'Hare, which it has tackled by rearranging departure gates to reduce on-ground aircraft congestion. At the same time, United has introduced high-tech devices it hopes will get passengers -- and their baggage -- into and out of the airport as smoothly and quickly as possible. Laser scanners, for example, now sort baggage automatically.

The design of the new, 85-acre terminal -- principally two long, parallel concourses topped with soaring glass ceilings -- emphasizes spaciousness, much as did the grand old railway stations of another era in transportation. "The concept is a monumental space to handle lots of people," says project architect Martin Wolf of the firm Murphy/Jahn. "Life is hard enough without buildings squeezing you." The size has enabled United to set aside smoking and nonsmoking areas.

Other airports, meanwhile, have introduced such creature comforts as shower rooms (useful after an overnight flight), health spas and even rotating art exhibits to help pass the waiting time. Several feature mini-offices for the business traveler who wants to keep busy.

The United project is part of a $1.6-billion modernization of O'Hare. Like other big city airports, O'Hare has struggled -- not always successfully -- to keep pace with the tremendous growth of air travel. Constructed in the 1960s to accommodate 20 million to 25 million passengers a year, today it serves twice that many. By 1995, it is expected to be handling 75 million to 80 million a year. United is the major airline at O'Hare.

To build its Terminal for Tomorrow, United has borrowed on recent ideas from other airports and come up with some of its own. For the first time, these latest developments have been integrated in a single facility. The result, says Rod Strickland, United's director of consumer services, is "the most modern terminal in the world."

Other airlines with new terminals may quibble with him, but there's no question that the new concourses are a big improvement over United's former, usually very crowded facilities.

Among the updated features at United's new O'Hare terminal:

Speedier aircraft turnaround. In the past, the arrangement of United's departure gates (designed for smaller planes and fewer flights) created a bottleneck, says Strickland. An arriving plane headed for an empty gate had to wait on the runway if another plane was departing any of the adjacent gates. In the new layout, the two parallel concourses where most planes will now dock are far enough apart so flights can arrive and depart simultaneously.

Another cause of flight delays is a mechanical problem with the aircraft. To cut down on repair time and get the planes into the air, United has installed a fully automated parts-retrieval system. About 40,000 spare parts are stored underground right at the terminal, enabling mechanics to obtain the part they need in 12 minutes or less. Before this, parts were delivered by van from a central warehouse. "It never took less than 30 minutes and upwards of that," says Strickland.

Passenger convenience. "Flow" was a key word in the design of the terminal. Passengers, it is hoped, will flow quickly and conveniently from curbside to departure gate. To accomplish this, United has done several things.

It has installed six curbside entrances, each with a curbside baggage check-in counter and its own baggage conveyer belt. Signs overhead indicate which entrance is most convenient to a particular destination. About 30 percent of United's passengers make use of curbside check-in. More may do so now that their luggage is loaded immediately on a conveyer belt rather than on a cart to be wheeled inside later.

Inside the terminal, United has placed what Strickland calls 56 "flow-through ticket counters." In older airports, ticket agents usually stand at a long counter with a wall behind them. United's new arrangement resembles, instead, the checkout islands of a supermarket. From the counters, passengers proceed directly to security checkpoints and on to the departure gate. Says Strickland: "You never have to make a U-turn." Or, he might have added, stumble over the bags of the passenger waiting in line behind you.

Passengers arriving by air are greeted as they exit the plane by an electronic sign that tells them on which of 14 carousels their luggage will be unloaded. As Strickland points out, frequent travelers play a baggage retrieval guessing game at some airports, scampering between unmarked carousels looking for the one with their suitcases.

Automation. There will be some big changes in ticket handling in the next year as United phases in automated ticket processing to its nationwide system. First to get the new process are Washington-Dulles (last month) and Chicago's O'Hare.

With their tickets, passengers at these two airports get a boarding pass with a magnetic strip on the back containing details about their flight. Anytime a passenger wants to make a change in a ticket, an agent can use the boarding pass to automatically call up the traveler's records. Strickland thinks this will be particularly useful in speeding up lines when a flight is canceled and dozens of passengers need new tickets.

About a year from now, passengers will enter their boarding passes at a "reading" device at the departure gate. This will help boarding agents keep track of who has boarded, and it should eliminate the confusion of two people being assigned the same seat. Nowadays, you don't know somebody else has also been assigned your seat until you get to it. Then you must wait for help from the attendant while other boarding passengers attempt to get around you. The "reading" device should catch the error before you board; the agent at the gate should be able to resolve the problem more quickly, and the flight won't be delayed because of it.

Baggage service. United has automated its O'Hare facilities, as it already has in Honolulu and Seattle and plans to do soon at Denver. In the old terminal, the carrier could move 75 pieces of luggage a minute manually; now it sorts 480 pieces a minute. Other airlines also are turning to automated baggage handling at large airports.

When passengers check in at United at O'Hare, each piece of luggage gets a sticker with a bar code (like the bar code indicating price on food products for automated grocery stores) that indicates the city where the luggage should be carried. The luggage is then loaded onto the conveyer belt, where laser-scanners direct it over a network of belts to the proper airplane. The result is less chance for error and faster delivery, especially important if you have a short connection between flights.

Later, when the magnetic boarding pass system is fully operational, a baggage clerk can enter the pass into a printer and produce an individually bar-coded tag for each piece of luggage that not only gives your destination but also spells out your name, flight number and the number of bags you are checking.

Electronic signs. United has coordinated all of its relevant computers at O'Hare so that all of the flight monitors provide the same arrival and departure information -- and that information is the most current available.

For example, Strickland explains, if a plane approaching O'Hare is put on a holding pattern by air traffic controllers, the pilot can enter a new estimated time of arrival in the aircraft computer. That information is relayed to United's on-ground computers, which immediately update the terminal monitors.

The agents' desks at the boarding gates also now have automated information signs, which will provide current scheduling times.

Ambiance: The terminal's planners wanted a facility that would be as interesting as it was functional, says Wolf, the project architect. So they made a big effort to enliven the 800-foot-long pedestrian tunnel that links the two parallel concourses.

Underground passageways can be dark, uncomfortable spaces, but this one attempts to delight with a neon sculpture that sends patterned pulses of colored lights the length of the ceiling. The lights are choreographed to a composition called "Tunnel Music."

The tunnel itself, which has four moving sidewalks, is enclosed in translucent glass, and behind it is an undulating colored wall. As Wolf explains, the soft tones coming through the glass give a sense of open space, so passengers are "not aware of where the boundaries of the tunnel are."

The hope is that the tunnel's decor will intrigue and entertain passengers en route to or from planes. "By the time they get to the end of the tunnel, they won't realize the amount of time they spent in it."

United's new terminal is part of a major overhaul at O'Hare that includes modernization at other airline departure gates, widening of the main roadway that leads to and from the airport and, within a few years, the construction of an automated people mover connecting outlying parking areas with the terminal area.

Certainly the project won't resolve all of O'Hare's many problems, and the Chicago Tribune already has raised at least one criticism of United's terminal. The new design makes a Chicago connection convenient only if a passenger is arriving and departing on United. "If he wanted to take an American plane," writes the Tribune, "he'd have to walk almost a mile to get to its terminal."

While the airline industry attempts to alleviate some of the causes of flight delays, a number of airports have introduced features to help you spend your on-ground waiting time more pleasantly and profitably. Among the diversions:

Sky-Tel. So far this 13-room mini-hotel inside the airline terminal is found only at the Los Angeles airport, but it may soon be duplicated at other major airports. "We're negotiating," says general manager Jeff Panish.

The compact rooms with bed and shower appeal to travelers who want to nap or freshen up while waiting for a connecting flight. They are particularly popular with business travelers arriving on overnight flights from the East Coast who want a chance to shower before hurrying off to an appointment.

For a fee that begins at about $10 (with tax) for a half hour and climbs to about $52 for an eight-hour stay, Sky-Tel provides an air-conditioned room, a small bed, a color television set, a desk, a telephone, a full-size shower and soap, shampoo, towels and toothpaste.

The rooms -- 6 by 13 1/2 feet -- are for single-occupancy only, although a child up to the age of 13 can accompany a parent. For information: (213) 417-0200.

Air Vita Club: The Dallas-Fort Worth airport got the first of these convenient health clubs three years ago. The second opened in Phoenix in December, and construction is underway on a third in Denver.

If you have time to kill in the airport, you can go for a workout in the facility's weight-training room, take a sauna or a steam shower, and enjoy a massage. In Dallas, the Air Vita Club has a year-round heated indoor/outdoor pool. In Phoenix, there are whirlpool baths.

At least one airline serving Dallas gives its passengers a choice of a meal or a visit to the Air Vita Club if their flight is delayed, according to manager Diane Rothe.

The basic fee is $15 whether you stay for an hour or all day. Razors, combs, hair dryers and assorted toiletries are provided. Workout clothing or swimsuits can be rented for $3. Massages are an additional fee.

The clubs are open Monday through Friday, 6:30 a.m. to 9 p.m.; Saturday, 10 a.m. to 6 p.m. and Sunday, noon to 8 p.m. For information: (214) 574-2026.

Tele-Trip Business Service Centers: These mini-offices for the business traveler who wants to keep busy are now operating in 13 major airports, including the three in the Washington area.

An affiliate of Mutual of Omaha, they provide a variety of services for varying fees. Perhaps the handiest for a frequent traveler is a private telephone suite with a small desk. Travelers can phone the home office or business contacts anywhere in the continental United States at a rate of $3.50 for three minutes and 45 cents each additional minute.

Other services include baggage storage, photocopying, secretarial help ($15 an hour), a conference room ($40 for the first hour), a notary, facsimile transmission and use of a computer. The offices generally are open seven days a week from 6 a.m. to 10 p.m., although hours vary by airport.

In addition to Washington, they can be found at airports in Pittsburgh, Detroit, Richmond, Atlanta, Fort Lauderdale, Norfolk, Los Angeles, Las Vegas, New York (LaGuardia) and Omaha. For information: (800) 228-9792.

Art exhibition: Artworks enhance the appearance of many airports, but the San Francisco Airports Commission goes a bit farther. Its exhibitions department stages up to 40 art shows a year from local and international collections. When waiting for a plane, you can browse the 17 exhibition areas as if you were in a museum