Friends and even lovers may find themselves occasionally squabbling on a vacation, so the possibility of discord seems sure to multiply when the travelers are separated by a wide age span -- for example, when grandparents take their grandchildren on a trip.

On the other hand, the youngsters and their grandparents often can learn a lot from each other and, given the opportunity of an agreeable journey together, may even become good companions.

With the latter in mind, a Chevy Chase travel agency, the Ticket Counter, is organizing a series of tours designed especially for grandparents and their grandchildren. The tours are scheduled on major school holidays from fall through next summer. They are aimed at children from about ages 7 to 14.

Among the initial trips scheduled are a Charles Dickens week in London over the Christmas holiday, Dec. 21 to Jan. 3; a long weekend at Disney World in Orlando, Fla., Feb. 13 to 16; and a week in Indian country in the scenic Southwest at Easter, April 11 to 19. Prices, as yet to be determined, will be all-inclusive (air fare, meals, lodging).

Planned for next summer are an Africa game park safari; a camping and sightseeing trip to Alaska's glacier country; a catamaran cruise among the Hawaiian Islands; a walk in New England; and a crafts tour (with instruction) in Ireland.

*"It's an idea which we think has really reached its time," says Helena T. Koenig, owner of the Ticket Counter, who has dubbed the new series "Grandtravel."

"There are so many children whose parents are both working, and the parents hardly have time for each other," she says. "And we've seen grandparents with some money and lots of time. They're interested in the teaching role, transferring their heritage." Many grandparents live at a distance from their grandchildren and often "feel disconnected, powerless and disoriented in what they define as their traditional roles as grandparents," she says. "Unfortunately, often their grandchildren suffer as well from a lack of support, nurturing and affection that in the past always emanated from their grandparents."

Koenig believes the tours will appeal particularly to grandparents who otherwise may see their children infrequently because of a divorce in the family. If they are -- understandably -- a bit shy with each other, they have the company of the other tour participants, and there's also a tour leader on hand to help smooth any differences.

One feature of each tour is a predeparture briefing designed to eliminate potential problems. "Grandmother might get tired, so we tell the children to have patience," says Koenig. "And Grandmother has to understand that they might want to watch TV at night, although she's spent all that money for their trip."

Koenig has organized the tours, especially the longer ones, so that the older folks and the younger ones have time alone with their own age group. One night the grandparents may all go dining at a gourmet restaurant, while their charges can enjoy themselves with a hamburger and a couple of hours of roller skating.

She has discovered she must organize the trips well in advance to give families a chance to reach a decision, a more complex process than arranging a family holiday. First, the grandparents must be interested in taking the youngsters; then the parents must agree to give them up; and, finally, the youngsters must be willing to go.

For information and a brochure: The Ticket Counter, 6900 Wisconsin Ave., Suite 706, Chevy Chase, Md. 20815, (301) 986-0790.

THE MIDDLE SEAT: Who gets the middle seat on an airline, universally regarded as the most uncomfortable place to fly? United Airlines has decided that, if possible, it shouldn't be their passengers who pay higher fares.

As of June 18, travelers using United's "ultrasaver" fares -- the ones that require a 30-day advance purchase -- no longer may select their seating in advance. They will be given a seating assignment only when they check in the day of the flight. About 20 percent of the seats are sold at an ultrasaver rate.

As United spokesman Joe Hopkins explains, passengers paying the cheapest fares -- which required advance planning -- could choose the best seats, window or aisles. The business traveler, often flying at the last minute and paying full fare, tended to get whatever seats were left over -- the middle seat, in many cases.

It was a choice, says Hopkins, of rewarding the advance planner or the passenger who paid the most, and United has decided to favor the latter. The company already has had complaints from budget travelers, but it used to get complaints from its business customers.

Hopkins sees the possibility that United's move may attract more business travelers, since they now have a better chance of getting a more comfortable seat.

EYES OPEN: A Washington traveler, returning from a trip abroad, warns of the possibility of theft at -- of all places -- the airport security station.

She placed her purse, containing her wallet with cash and credit cards, on the X-ray conveyer belt. She then stepped through the metal detector, but a security guard ordered her to do so again.

While she was distracted, she says, her wallet was removed from her purse. She is certain she had the wallet before she passed through security because she had just paid an airport departure tax at flight check-in.

The moral, of course, is that travelers should keep a close eye on valuables, even when they least suspect trouble.

PARKS PASSPORT: Here's a new idea for fans of America's national parklands, both a way to learn about the parks and help finance park interpretive programs. This is the way it works:

For $2.95, you can buy a "Passport to Your National Parks," a 104-page pocket-sized booklet listing all 337 national parks, monuments, memorials, historic sites and other park preserves. There are also nine regional maps and one national map showing park locations.

Beside each park listing, there is a blank space. When you visit a park, a ranger will validate the passport, entering the date much as immigration officials do with international passports. It is a way to record you and your family's travels, especially if you are a national park collector.

The passport, inaugurated in May, is good for five years. In addition to the validation, you can also obtain art stamps to paste into the passport. There will be a national stamp for each of five years. This year's stamp features the Statue of Liberty and sells for $1.

Each of the nine regions also has an art stamp, and a new one will be issued every year for five years. Each costs 50 cents. This year's stamp for the National Capital Region displays the Vietnam Veterans Memorial.

You can buy a national stamp at any park site. To get the regional stamp, you must visit a park in the region. Each year's stamps also are available by mail after Oct. 1.

The passport was created by the Eastern National Park and Monument Association, one of 67 nonprofit associations aiding national park programs. The association has printed about 250,000 copies, and sales reportedly are brisk. The income will be turned over to the park system.

In addition to bringing in money, the association hopes the passports will help acquaint more Americans with the wide diversity of the nation's parklands, particularly the less-visited ones. A traveler visiting a well-known park can consult the booklet for the location of other nearby parks.

In Washington, the passports are available at national park sites on the Mall. They also can be ordered by mail from the Eastern National Park and Monument Association, Jamestown National Historic Site, Jamestown, Va. 23081.

For information: (800) 821-2903.