"SPECTRUM: Natural Settings," a contemporary show of seven artists -- painters, sculptors, ceramicists and photographers -- is a thought-provoking, entertaining exhibition that explores modern attitudes toward landscapes; it's free and worth checking out. CLASSICAL MUSIC

The viola da gamba, that noblest of obsolete musical instruments, will take the spotlight Saturday night at the United Church on G Street NW, when the New York Consort of Viols visits Washington under the auspices of the D.C. chapter of the Viola da Gamba Society. The program will include English and Italian music of the late Renaissance and early Baroque periods. Angela Cheng, winner of the 1984 University of Maryland Piano Competition, will give a recital today at the Phillips Collection. Other highlights of this afternoon's music include Paul Hill's Washington Singers with organist Eileen Guenther at the Foundry United Methodist Church and the Cleveland Park Chamber Concerts at the Art Barn in Rock Creek Park. This evening, violist Kim Kashkashian will be presented in the Pro Musicus Series at the Renwick Gallery, and lyric mezzo-soprano Rosemary George will sing at the National Gallery.

Conductor Yoel Levi and organist William Neil are the National Symphony's guest artists this week. The program will include the Third Symphony of Camille Saint-Saens.

Composer-pianist Russell Woolen will be heard in both roles in this Friday's lunch-hour free concert, sponsored by the Friday Morning Music Club at the New York Avenue Presbyterian Church. Other noteworthy chamber music of the week will include the Alexander String Quartet, Friday night at the Wolf Trap Barns, and wind chamber music by Mozart, Ligeti, Janacek and Beethoven in the Music from Marlboro series, Saturday afternoon at the Library of Congress. DANCE

The Houston Ballet ends its run of "Swan Lake" this afternoon at the Kennedy Center Opera House. The company resumes Tuesday evening with a repertory program featuring the world premiere of director Ben Stevenson's "The Miraculous Mandarin" and the American premiere of Joe Layton's "The Grand Tour." Stevenson's full-length "Peer Gynt," to the music by Grieg, receives its Washington premiere Wednesday evening in the first of four performances. The company's visit ends next Sunday afternoon with "Peer Gynt."

The Dance Company of Middlebury, a modern dance troupe sponsored by Vermont's Middlebury College and directed by faculty member Andrea Olsen, appears Friday night at Alexandria's Lee Center, in a program of choreography by Olsen and others. FILM

Thursday at 7:30 p.m. at the Library of Congress' Mary Pickford Theater, Richard Coe, critic emeritus of The Washington Post, will talk about "Changing Styles of Shakespearean Film Acting," with silent-era "Richard III" and "Cymbeline," a six-minute segment of John Barrymore from "Henry VI, Part One" and a videotape of Ian McKellen's stage show, "Acting Shakespeare."

Saturday at 6 p.m. at the American Film Institute, "Harper," a Lew Archer mystery starring Paul Newman. With a terrific script by William Goldman, based on the Ross MacDonald novel.

Among current releases, Akira Kurosawa's "Ran," an impressive achievement from the Japanese master. POP MUSIC

Violinist John Blake has slipped into a crossover stance on his new record; live at Blues Alley on Monday, he's more likely to showcase the vivid string-bound explorations that marked him as a major new voice in jazz.

Joe Ely was one of the original cowpunks, using a great band to create a raucous, eclectic mix of Tex-Mex, West Texas and Louisiana styles. On his last album, he went over the wall to wholly synthesized music. On Wednesday, he'll perform solo at the 9:30 club, and your guess is as good as his. THEATER

Just when you thought everything that could be said about Richard Nixon had been said, along comes "Secret Honor" (New Playwrights' Theater), which claims the former president purposefully engineered Watergate to save the republic from a right-wing conspiracy. Cockamamie as it may sound, actor Philip Baker Hall, playing a boozy, vindictive Nixon, makes this one-man play funny, frightening and oddly moving.

"Sally and Marsha" (Round House Theatre) is about the forging of an unlikely friendship between a city mouse and a country mouse, who meet in a New York apartment building. Sybille Pearson's appealing comedy is turned into something special by the splendid performances of actresses Kathy Yarman and Gina Franz.