USAir offers at least one flight daily from Washington, D.C., to MacArthur Airport in Islip, Long Island. Round-trip fare is $125 if reservations are made within seven days of departure. The nearest Hampton is Westhampton Beach, a 30-minute drive; a car, which can be rented at the airport, is a must.

Motels are commonplace along the main highways, and there are some charming restored inns which offer accommodations through the fall. At the American Hotel in Sag Harbor, an old whaling village, the rooms have been remodeled in keeping with the original 19th-century decor. A double room with bath costs $60 during the weekday, $80 for a two-night weekend.

The Maidstone Arms on the Village Green in Easthampton has a wing dating back to the 17th century. From Oct. 1, the room rates for a double, depending on whether you share a bath, are from $45 to $75 including a continental breakfast. Both the American Hotel and the Maidstone Arms have excellent restaurants and serve meals during the week, as well as on weekends. Hours and days of operation at other restaurants may vary during the fall season.

Sip N Soda in Southampton, The Candy Kitchen in Bridgehampton, and the Paradise Restaurant in Sag Harbor are local luncheonettes specializing in homemade ice cream and catering wholesome food to budget-minded families.

Gurney's Inn, a Montauk spa overlooking the ocean, is equipped with a sauna, steam and exercise rooms and an enclosed saltwater pool, all of which is available to nonresidents for an $11 fee.

Some of the descendants of the mammals caught by the Sag Harbor whalers may be seen on a whale-watching expedition sail out of Montauk on October weekends, weather permitting. A biologist and naturalist are on board to lecture and answer questions. Cost: $15 for children, $25 adults. For information and reservations write to the Okeanos Ocean Research Foundation, 216 E. Montauk Highway, P.O. 776, Hampton Bays, N.Y. 11946.

The South Fork-Shelter Island Nature Conservancy conducts nature walks throughout the Hamptons and Montauk every Saturday morning until Dec. 19. Reservations are required and the charge is $6. Call the Conservancy office between 9 a.m. and 5 p.m. at 516--267-2748.

At Hidden Echoes Ranch, also in Montauk, you may rent horses to ride along the beach for $10 an hour. The horses are old, slow, steady, and led by a guide; no previous equestrian experience is necessary.

The South Fork and indeed Long Island itself ends at the Montauk Lighthouse, which rests on a rocky promontory. Above the beach is the Montauk Point State Park, reached by a number of winding footpaths. It has picnic tables, a snack bar, and a telescope -- for a small charge you can look out on the nearest landfall, Block Island, R.I.

Two books worth reading about the area are Everett T. Rattray's "The South Fork, The Land and the People of Eastern Long Island" (Random House, $10), an anecdotal history drawing on old records, correspondence, diaries and family folklore; and "A Guide to Sag Harbor," by Henry Weisburg and Lisa Donneson (St. John's Press, $3.95), a walking tour of the town with detailed descriptions of its picturesque 19th-century houses.