"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 landscape. The Corcoran Gallery show is free and worth checking out. CLASSICAL MUSIC Pete Seeger will be the guest artist of the Washington Philharmonic today at the Metropolitan AME Church, in a program that will include folk music and classical compositions (by Ives and Vaughan Williams) based on folk music.

Rafael Fru hbeck de Burgos replaces the ailing Klaus Tennstedt as guest conductor of the National Symphony this week. The program (Wagner's "Siegfried Idyll" and "Meistersinger" Prelude, and Beethoven's "Eroica") will remain unchanged.

The Kennedy Center's Handel Festival will open its 10th season with the same work that opened its first: the highly dramatic oratorio "Saul." Bass-baritone Morley Meredith in the title role and music director Stephen Simon as conductor will repeat their performances of January 1977. Other performers in the well-chosen cast will include Linda Mabbs, Paul Esswood, Elaine Malbin, James Atherton and John Stewart.

Also from the 18th century:

Mozart's matched pair of contrasting serenades, "Eine Kleine Nachtmusik" and "A Musical Joke," presented by the Smithson String Quartet with guest artists, Tuesday and Wednesday nights in the Hall of Musical Instruments at the Museum of American History.

Music of Vivaldi and Corelli, performed by Trevor Pinnock and the English Concert, Wednesday night at the Kennedy Center.

Washington composers will be featured in tomorrow night's program presented by the Contemporary Music Forum at the Corcoran Gallery.

And Washington composer John Philip Sousa will be featured in two concerts Friday and Saturday nights at the Wolf Trap Barns. The conductor of these programs (played in uniforms that replicate those of the original Sousa band) will be the well-known Sousa scholar and interpreter Keith Brion. The program (being taped for the PBS "On Stage at Wolf Trap" series) promises to be spectacular.

Also spectacular will be a marathon eight-hour concert of "Music for a World Without Hunger" to be presented by Catholic University students and faculty from 4 p.m. to midnight Friday in the Ward Recital Hall.

Pianist Anne Koscielny will be the soloist, with George Manos conducting the National Gallery Orchestra, tonight at the National Gallery in a program of Ravel and Debussy.

Some of the world's greatest saxophone players will be at the Navy Band's Sail Loft, Friday and Saturday for the band's saxophone symposium.

Noteworthy chamber music of the week: cellist David Hardy, Saturday night at the Dumbarton Church; recorder virtuoso Phyllis Lanini, Saturday night at the Corcoran; the Amore Trio, this afternoon at the Bethune Museum-Archives; the Aspen Trio, this afternoon at the Phillips Collection; the Trio Mexico, Wednesday night in the Hall of the Americas at the Organization of American States. DANCE

Lesa McLaughlin and Dancers, one of the liveliest of area troupes, performs three premieres and other repertory at the Dance Place this afternoon. On Wednesday, the Houston Ballet begins a two-week engagement at the Kennedy Center Opera House; the first week's programs are devoted to the troupe's full-length production of "Swan Lake," to be seen in its Washington premiere with two casts of principals. The Joffrey II Dancers appear at George Mason University's Harris Theatre Wednesday night as part of an International Arts Festival, in a program featuring a premiere by Ray Powell, a Bournonville "Divertissements" staged by the late Toni Lander, and works by Philip Jerry and Lynn Taylor-Corbett. Mark Taylor and Friends, a New York-based troupe, will give the last performances at Dance Place's present quarters in Dance Alley Friday and Saturday evenings, before the organization's impending move to the Brookland area. Also Friday and Saturday nights, Arlington Dance Theatre, a community-based troupe, presents a 30th-anniversary concert at the Thomas Jefferson Theatre featuring choreography by resident and guest artists including Liz Lerman, Rex Bickmore, Adrian Bolton, John Lechner, Marilyn Mazur and Clovia Chinn. POP MUSIC Not surprisingly, there are several major musical tributes to the Rev. Martin Luther King Jr., whose national holiday recieves its first official celebration tomorrow. Today, Jennifer Holliday and Jeffrey Osborne share the bill at the Warner (with proceeds earmarked for the King Mural Project), while Pete Seeger joins the Washington Philharmonic at the Metropolitan AME Church (with proceeds assisting in the maintenance of the Philharmonic and the Urban Philharmonic Society).

On Monday, the Kennedy Center hosts a gala that will feature Stevie Wonder, the man who pursued the vision of a King holiday to fruition, as well as Diana Ross, Quincy Jones, Eddie Murphy, Bob Dylan, Peter, Paul and Mary and the Alvin Ailey Dancers. The show will be taped and parts of it will appear later Monday night on an NBC special, along with concert material from New York and Atlanta.

The last time he was in town, Dee Snider was testifying on Capitol Hill against lyric labeling. On Wednesday, he'll join the rest of Twisted Sister for some musical mischief at the Capital Centre.

Two folk events occupy Saturday: the Folklore Society of Greater Washington's annual winter mini-festival takes place at Takoma Park Junior High, starting at noon and featuring a cast of dozens that includes Ed Trickett, Bob Hitchcock, Joe Glazer, Mary Cliff and Joe Hickerson, and culminating in a dance and sampler concert in the evening.

That evening, Lisnerauditorium is the scene of a tribute to WLTT deejay Dick Cerri and his 25 years of radio programming and championing of folk music of all kinds. This, too, features a cast of dozens, including Schooner Fare, Christine Lavin, Jonathan Edwards, the Seldom Scene and Carolyn Hester. THEATER Just when you thought everything that could be said about Richard Nixon had been said, along comes "Secret Honor" (at the New Playwrights' Theatre), 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" (at the 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.