Famous sites and monuments and the Cherry Blossom Festival Parade will highlight a trip to the nation's capital beginning April 4 for four days. On a tour of Washington, D.C., participants will visit the Capitol Building, Supreme Court, Library of Congress, National Archives and Vietnam Veterans Memorial. In addition, plans call for viewing the changing of the guard at the Tomb of the Unknown Soldier, the Kennedy gravesites and the Challenger Memorial, plus the Kennedy Center for the Performing Arts and Embassy Row.

Also, the tour will stop at the Smithsonian Institution, Museum of Natural History, Museum of American History and the National Air and Space Museum. Cost is $289 per person, including motor coach, accommodations, sightseeing and gratuities for luggage handling. For more information, contact Domenico Tours, 751 Broadway, Bayonne, N.J. 07002, (800) 554-8687.-0

A motorcyclist's view of Thailand is being offered April 10 to April 24 by Von Thielmann Tours, P.O. Box 87764, San Diego, Calif. 92138.

The Grand Palace and the Temple of the Emerald Buddha in Bangkok, entertainment along the beach road in South Pattaya, a boat ride to Coral Island, explorations of the cities of Ayuddhaya and Sukhothai, a view of Buddha's footprint in Saraburi, plantations of fruits and other products, an elephant training camp, hill tribe villages, the "monkey" capital of Thailand are on the schedule.

Cost is $2,758, including round-trip air fare from Los Angeles to Bangkok, plus accommodations, transfers, most meals, tour guides and a mechanic and a luggage carrier. Participants can choose either motorcycles or a mini-bus, with motorcycles in three different sizes offered for rent.

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Cambridge, Idaho, will be the start of rafting trips on Snake River in Hell's Canyon being planned by Idaho Afloat from May 18 to Sept. 27 in the Pacific Northwest. Two-, three- and four-day trips for people from 7- to 83-years-old are available from $330 to $700, with breakfasts, lunches and dinners part of the price. Activities include hiking, photographic opportunities, fishing and swimming, plus visits to archeological sites, Indian burial grounds and abandoned homesteads. Participants can choose between traditional oar rafts operated by guides, paddle rafts or kayaks. For more information, contact Idaho Afloat, P.O. Box 542, Grangeville, Ida. 83530.

Prices, dates or itineraries should be confirmed with airlines,

cruise lines, tour operators or travel agents.