It ain't the Beach Boys, but on Sunday, the Boston Pops will be giving an early-evening, free concert on the steps of the Lincoln Memorial. Conductor John Williams is bringing the famed orchestra to Washington as part of a 14-city tour to celebrate the group's 100th anniversary. For those missing the outdoor concert, it will be seen over the PBS network later that evening. Singer John Denver and a 350-member chorus made up of members from local choirs will perform with the Pops. The following evening the Pops will perform for President Reagan and Nancy Reagan and the U.S. diplomatic corps at the White House . . .

Washington's premier jazz pianist John Eaton is bringing together a group of his friends to join him for a live recording session this week at the Embassy Row Hotel. Called appropriately enough "John Eaton and Friends," the performances will be recorded during his Thursday, Friday and Saturday night shows there. His friends appearing with him are clarinetist Wally Garner, bassist Tom Cecil, jazz vocalist Linda Cordrey and soprano Alessandra Marc . . .

After 3 1/2 tons of sand, several cans of spilled beer and fish were thrown around the stage for Steppenwolf Theatre's production of "Coyote Ugly," the Kennedy Center's Terrace Theater has been cleaned up and sanitized for "Kabuki Medea," which opens Saturday. In fact, there will be no shoes worn on stage, even by the stagehands . . .

Rock singer Billy Joel has announced he will donate royalties from a new record titled, "You're Only Human (Second Wind)," a song about teen suicide, to the National Committee for Youth Suicide Prevention. He is interested in the problem because as a teen-ager he once came close to suicide . . .

A Los Angeles Municipal Court judge yesterday vacated contempt of court orders against two writers who had refused to testify about interviews with Cathy Evelyn Smith in which she reportedly admitted administering the drug dose that killed John Belushi . . .

Penthouse magazine may be planning a major layout of sexually explicit pictures of rock singer Madonna, but she's not going to get upset about it. A spokesman for the singer said she acknowledged that she posed nude many times for art classes, probably in her late teens while she was a student, and "doesn't feel she's done anything she's ashamed of." . . .

Yesterday's 8 a.m. USAir flight from MacArthur Airport in Long Island to Washington didn't get off on time and all those powerful Washingtonians were stuck for two hours because the boarding stairs were frozen halfway up. At one point former Cabinet official Joseph Califano, restless with the delay, announced he was nominating fellow passenger former senator Eugene McCarthy "to be the spokesman for the hostages." . . .