TRAVELERS who have flexibility in their plans often can benefit from money-saving deals designed to fill up empty airline seats and hotel beds. Three U.S. firms have put together intriguing offers aimed at cutting the cost of both business and vacation trips.

* Discovericard: The idea here is that the traveler pays full rate for one night in a hotel and gets the second night free. The upfront cost is $30 a year for an identification card. Members show it at any of more than 300 participating hotels to get the two-for-one price.

Accommodations are available in the country's top 100 business cities and top 40 vacation destinations, says Timothy Gaffney, president of Discovericard of Hartford, Conn. Generally, there's a choice of top, medium and budget-range hotels. Based on Mobil travel guide ratings, most are either two-star ("very good") or three-star ("excellent"). A copy of Mobil's cities guide is included in the sign-up fee.

A directory listing participating hotels is sent to card holders twice a year. Another 100 hotels are expected to be added when next January's directory is printed.

Currently, the hotels in Boston include the Copley Plaza downtown and a suburban Ramada Inn. In New York City, it's the Milford Plaza and the Beverly in Manhattan and the Travelodge at J.F. Kennedy International Airport. Resort hotels include the Topnotch at Stowe, Vt., and the Marriott at Disneyland in Anaheim and Disney World in Orlando. Locally, the L'Enfant Plaza Hotel is listed.

The catch is that the bargain is restricted to a hotel's off-peak periods, though on an average, says Gaffney, the offer is available 200 days a year. Open dates vary by hotel and may, or may not, include weekends.

Why the cut rates? "Hotels are in the same position as airlines," says Gaffney. "Empty seats produce no income. Here they are getting a full night's revenue, and they have a captive audience for the second day to market their lounge and restaurant."

For information: Discovericard Inc., P.O. Box 1984, Hartford, Conn. 06144.

* Holiday Condominiums: This is a possibility for travelers who put off decisions until the last minute or for vacationers for whom a condominium apartment is an agreeable alternative to a hotel room.

Vacationers making reservations no more than 21 days in advance are promised a 30 to 40 percent discount on condominium rentals at resorts in the United States and around the world, at least through the end of this year.

The program is patterned after airline standby offers, says Elliott S. Hechtman, president of Holiday Condominiums Inc. of Omaha. In Holiday's standby deal, prices for unbooked condos are discounted at the last minute as a way to fill them.

In business since 1980, Holiday serves as a clearinghouse for about 32,000 condo units, including studio and one-, two- and three-bedroom apartments. If the normal rental price is more than $500 a week, the discount is 40 percent. At $500 and below, the discount is 30 percent.

Holiday's heaviest concentration of units is in Florida, particularly the Orlando area, along with Hawaii, Mexico and the Carolinas. Reservations can be made by phone or through a travel agent. In August, a variety of units were available in Orlando at a prediscount rate of $450 to $700 a week, accommodating six to eight guests.

For information: Holiday Condominiums Inc., 7701 Pacific St., Suite 300, Omaha, Neb. 68114 (800) 228-0002.

* Traveltron: This is a new, California-based airline reservation agency that promises to search out the cheapest airfare between destinations in the United States. The service is free, and Traveltron makes its money if you decide to book your flight with the firm.

The complexities of the U.S. airfare structure are well-known. On one day last month, says Traveltron vice president Tom Davis of Irvine, there were 40,000 changes to fares and schedules. The firm claims its array of computers and its specialized staff, which--unlike other travel agencies--handles only airline reservations, can keep abreast of these fluctuations. Because of the growing demand for rate information, the staff was tripled this month.

Traveltron also promises to seek out "creative" routing if that is cheaper. One day this month, it quoted a Washington-to-San Francisco roundtrip fare of $318, via Piedmont Airlines to Charlotte, N.C., and Dallas, with a connecting American Airlines flight to San Francisco (depart National at 10 a.m.; arrive in San Francisco at 5:37 p.m.). That compared with the cheapest nonstop supersaver fare of $356 (with comparable advance-purchase and length-of-stay restrictions) and an unrestricted coach fare of $838.

Travelers with flexibility who can take unusual routes, travel midweek or nights and book at least a week in advance have the best cahnce of getting the cheapest rates, says Traveltron agent Richard Russo. He recommends paying for the ticket immediately because any rate could jump the next day.

For information: (800) 221-7041. DIAL-A-PARK: What's the weather forecast today in Yellowstone or Grand Teton national parks in Wyoming? Dial (900) 410-PARK for a recorded message. The cost is 85 cents for two minutes, plus taxes.

During August, the National Park Service and the National Park Foundation are conducting a test to see if a dial-a-park service is a practical way of providing up-to-date information to prospective visitors.

The two Wyoming parks were selected because they are among the nation's most popular. Last August, 720,600 vacationers toured Grand Teton; at Yellowstone the figure was 626,800. The trial run ends after Aug. 31.

If the service proves helpful, it could be expanded next year to other popular parks, such as Shenandoah, Smoky Mountains and Yosemite. Currently, park information is obtained by calling the park directly, usually at a higher cost and with the very real possibility of getting a busy signal.

Along with the weather, the record message includes information on park programs, entrance and camping fees and--perhaps most important to many visitors--details on campsites available. In one morning's message, the service advised that one of Yellowstone's most popular camping areas had filled--on a first-come, first-served basis--at 10:25 a.m. but another had vacancies until 6 p.m. At Grand Teton, the first camping area filled at 7 a.m.

In a park as large as Yellowstone--2.2 million square miles--that is important to know, says Interior Department spokesman Duncan Morrow. "A lot of campgrounds may fill up rapidly. If you don't know that, the one you go to may be filled and the next closest one to it in the park may be 70 miles away." CALLING HOME: About 63 percent of Americans vacationing abroad phone home at least once and the figure jumps to 83 percent for business travelers. A lot of them are shocked to discover their hotel has imposed a surcharge that doubles or even triples the cost of the call.

The American Telephone and Telegraph Company, which has been working to control excessive surcharges overseas, cites these examples:

* A talkative California businessman phoned his Oakland office from a hotel in Barcelona, ringing up a phone-company charge of $120. The hotel billed him for $500.

* A tourist from Peekskill, N.Y., made a $64 phone call home from Munich. The hotel's charge was $352.

* A bureaucrat phoning Washington from Beirut found a $416 hotel surcharge added to his $270 call.

When they get back home, these travelers complain bitterly to the phone company. As a result, AT&T developed Teleplan. Under the program, the phone company attempts to get agreements with foreign hotels to limit surcharges to a "reasonable" level. So far, 900 hotels have signed, but that is less than 1 percent of registered hotels worldwide.

Teleplan agreements have been reached with hotel associations of Ireland, Israel, Panama and Portugal; Hilton International, Regent International and Marriott hotel groups; Golden Tulip Hotels of Holland; Lygon Arms Hotel in England; the Ledra Hotel in Cyprus; Comfort Hotels International in Britain, France and the Netherlands; Inter-Hotels in Switzerland; Mandarin International in the Far East; and Novotel Hotels in Germany and Austria.

Depending on the participating hotel, the agreed upon surcharge per call can range from 20 cents to a maximum of $10.

If you are not staying at a Teleplan hotel, AT&T suggests making a quick call home, leaving your hotel and room number, and then having the home party return your call. Generally, the hotel surcharge is imposed only on outgoing calls, with the rate determined by the length of the call. Also, international rates on calls made from back home tend to be cheaper than those abroad.

Also, it's usually cheaper to dial directly than to have the hotel operator do it for you.

Says AT&T: A tourist in Paris was charged $95 for a five-minute call from her hotel room to New York City. Had she phoned from a Paris post office, another alternative, it would have cost about $18. A pre-arranged, direct dial return call from her home or office in New York would have cost only $8.50.