Undated (AP) _ Celebrations for the 200-year-old piece of parchment that became the U.S. Constitution highlight a Memorial Day weekend that will also be marked by the snarl of racing engines, the sweet sound of jazz and the rustle of millions of beach blankets on seaside sand.

Good weather and spring fever brought outdoor activities to the fore, with everything from inner-tube river races to kite-flying on the programs. Beaches and national parks were bracing for millions of visitors.

Veterans groups across the nation were planning Memorial Day observances shadowed by the deaths of 37 Navy sailors in the Persian Gulf on Sunday. President Reagan attended a service Friday at Mayport Naval Station, Fla., for the men who died when an Iraqi plane attacked the USS Stark.

Whether by land, sea or air, it was a sure bet travelers would not be lonely. Washington state officials warned of clogged ferries and overflowing airport parking lots and said aircraft would target speeders for a beefed-up squad of state troopers.

''In some cases, the travel increases by as much as 50 percent, with people visiting families, relatives, or going home from school over the weekend,'' said Howard Graff of the New Mexico Traffic Safety Bureau. The peaks, he said, will probably be late Friday and late Sunday.

The Memorial Day weekend was the first holiday test of the new 65 mph speed limit, and Utah police worried motorists would not notice the limit is still 55 mph in urban areas. For safety's sake, many states were sponsoring The Great American Buckle-Up.

The Spoleto Festival U.S.A. opened with a bright banner and brass fanfare Friday at Charleston, S.C., City Hall, where a bust of festival founder and artistic director Gian Carlo Menotti was unveiled.

Richard Strauss' ''Salome'' was bowing Friday night. B.B. King's blues and a performance by 19th-century marionettes were other highlights among more than 100 performances scheduled from now to June 7.

The Constitutional Convention opened 200 years ago Monday, and Philadelphia kicked off a party with Friday night fireworks and a concert by jazz saxophonist Grover Washington Jr. The holiday weekend festivities are expected to draw more than a million people, including Vice President George Bush.

In Boston, the USS Constitution, which is almost as old as the U.S. Constitution, will fire a 21-gun salute Monday to honor all veterans of American armed forces who died serving the country.

In Houston, organizers hoped 200,000 people would take part in a Saturday march to honor Vietnam veterans. A traveling replica of the Vietnam Veterans Memorial, bearing the names of 58,132 who died, was dedicated Thursday.

Branch 162 of the Fleet Reserve Association planned to cast wreaths into the Mississippi River at Gretna, La., and in Delaware the Wilmington and Western Railroad was offering half-price fare Sunday to all veterans in uniform.

No War of 1812 markers were available when Hugh McCallister died in 1882 at 90, so his grave in Williamsburg, Iowa, was marked with a Civil War standard. He had 22 children, and this weekend several of his descendants are setting up the right marker.

The world of sports was crowded with everything from the NCAA Division II World Series in Alabama to the Indianapolis 500, where 33 cars will roar around a speedway oval Sunday before a crowd of 400,000. On Saturday, Lambchop, Mickey Mouse and ''The Beaver'' - Jerry Mathers - were to headline the ''500'' Festival Parade.

An outwardly more sedate event was on tap at Wildwood, N.J, where 90 kite- fliers are competing in the East Coast Stunt Kite Competition.

The pilots, as the kite-fliers call themselves, make the kites dip and dive at up to 90 mph in individual and team routines. Kite manufacturers will show off their latest models, which cost as much as $200.

In Honolulu, arrival ceremonies were set Saturday for the Hokule'a, a 60- foot Polynesian sailing canoe that completed a two-year South Pacific voyage without modern navigational instruments.

Another group of sailors was still on an equally unusual odyssey. After towing a garbage barge to Central America and back, they were going to spend the weekend in New York harbor waiting for a place to dump their 3,100 ton load.

An inner-tube regatta was set for the Genessee River in upstate New York and Montgomery, Ala., girded for Friday's Whoopeedoo Parade, a come-one, come- all affair of whacky costumes and marching imprecision.

In Louisiana, the tourist could visit the state Special Olympics Summer Games in Hammond; the 12th annual Sawmill Festival in Fisher; and a candlelight vigil for AIDSvictims in New Orleans.

Memorial Day crowds of 200,000 were expected at Ocean City, Md., with 120,000 people expected at beaches in Delaware. In New York, 1.2 million people were expected to make a trip to one of the city's three major beaches. Fifty new lifeguards were sworn in Friday to help handle the jam.