Isreali Prime Minister Menahem Begin brought such high hopes and good humor to his lunch break yesterday at the Israeli ambassador's residence that one wonders what sort of grand celebration the Israelis might hold if the Mideast peace efforts succeed.

"It's so upbeat, such an exciting time," said Vivian Dinitz, wife of Ambassador Simcha Dinitz, shortly after Begin stepped in from the rain right on schedule. "We're punctual," he said, smiling broadly. "We are simple folks. Polite we should be."

Begin greeted about 40 guests with handshakes and hugs, leaving them somewhat dazzled by his extraordinary mood following his meetings with President Carter, who declared support for the Israeli peace proposals.

"The prime minister told me I tooked 18" said Rhea Schindler, wife of Rabbi Alexander Schindler, head of the Conference of Presidents of Major American Jewish Organizations. "First he told me I looked 24, then he dropped it to 18. And I have a 20-year-old daughter. he was ebullient."

Begin was ushered to a chair, but the 64-year-old prime minister wasn't seated long before he sprang up to greet AFL-CIO president and friend of Israel George Meany, who is 81. (The two share the same birthday, Aug. 16) "Mr. Meany, it's wonderful to see you," said Begin. "We are very grateful to you, Mr. Meany."

A platoon of White House officials was gathered about including Robert Lipshutz, counsel to the President; Zbigniew Brzezinski, national security affairs adviser, and Stuart Eizenstat, domestic affairs and policy adviser. About midway in the cocktail hour, Secretary of State Cyrus Vance was shown in by Mrs. Dinitz.

Among others present were Samuel Lewis, U. S. ambassador to Israel; Samuel Rothberg, chairman of the Israel Bond Organization; violinist Isaac Stern, and television news personalities Walter Cronkite and Barbara Walters.

Before going into lunch, Ambassador Dinitz declared Begin to be "in an excellent mood" and "very satisfied" with his talks with Mr. Carter. After eveyone had been seated, presidential assistant Hamilton Jordan showed up and was hustled in by a scurrying aide.

After the meal, which was closed to reporters, Begin visited the home of Sen. Hubert Humphrey (D-Minn.) at Harbour Square in Southwest Washington. After they met, the two appeared in the lobby, and Begin called Humphrey "one of the greatest friends of Israel . . . All throughout the years he's helped us."

Humphrey said that after their discussion he believed the "chances for peace are very, very good," adding. "A real settlement between Israel and Egypt would be the most significant event of the 20th century."

With that, a still-smiling Begin left Washington for New York and a meeting today with U.N. Secretary General Kart Waldheim.