Leonard Bernstein gave Sargent Shriver a classy birthday party at the Watergate last Friday night, with steak and lobster for 30 people and an informal musicale that went on until 7 a.m.

Everyone had such a good time that almost nobody noticed that none of the 30 people present was Sargent Shriver.

Lenny's friends suspect that maybe he has been crowding so much into the fun-filled hours he has spent in Washington during the past two weeks, that he might have forgotten to invite Shriver.

He has had a busy, busy time, and enjoyed it so much that he is looking for a house here.

Earlier on Friday, Bernstein chartered a plane out of National Airport and flew over to the Eastern Shore to go estate-hopping with close friend Washington stockbroker Bill Cook. They lunched with Jody du Pont on her yacht and Cook gave Lenny a cowboy hat banded with pheasant feathers to keep the sun out of his eyes.

The many-talented Bernstein even flew the plane back from the Chesapeake Bay outing. It was his second lesson, he told the pilot, who gave him the controls of the twin-engine Aero Commander. He didn't say when he'd taken the first.

Cook arranged the Shriver party for Bernstein after they winged back here in time for Lenny to conduct the Vienna State Opera's performance of Beethoven's "Fidelio." The Watergate, in addition to sending the chief out to shop for special food, was very accommodating about moving a piano into the private dining room.

The party eventually adjourned upstaris to Bernstein's private apartment, which is terribly chic, except for little signs on the furniture in the drawing room which proclaim: "Please use ashtrays. This is an antique."

When Bernstein went to the Pisces Club after his first performance here last month, he left early when no one could find him a piano. He took everyone in his entourage back to Cook's Georgetown house, where he gave them a private concert that lasted until 4:30 a.m.

Neighbors who couldn't have been music lovers called the cops to complain about the noise. Instead of quieting the party, the two officers who answered the call sat down to listen.

Hamilton Jordan's buddy, John Golden, appears to be functioning as a kind of one-man vigilante committee to round up Carter administration defectors and try to bring them back if he can. When Nick Friendly left the SBA to run Sen. Edward M. (Ted) Kennedy's campaign in Oklahoma, Golden was on the phone as soon as the defection was known. He tried in vain to get Friendly to give him names of others who might be planning to leave soon. . . .

Suzannah Lessard turned down $2,000 from the Washingtonian magazine for her piece on Ted Kennedy's philandering and gave it to the proud-but-poor Washington Monthly . . . Out in Las Vegas, a public relations man for the MGM Grand Hotel is promoting Ol' Blue Eyes' hard-drinking friend, Dean Martin, as "Ol' Red Eyes."