This Memorial Day weekend offers myriad things to do and see. At Arlington National Cemetery there's a presidential wreath-laying ceremony at the Tomb of the Unknown Soldier at 11. If you're taking Metrorail, exit the Blue line at the Arlington National Cemetery station. And at the Vietnam Memorial (near the Lincoln Memorial), there's a two-hour commemorative ceremony starting at noon. * Back over in Old Town, there's a special Memorial Day music festival featuring the sounds of Doc Dikeman and his Big Band, Kaleidoscope, Explorer Jazz and the Navy Commodores. It all starts at noon on Market Square, with performances on the hour until 6. Listeners are encouraged to bring lawn chairs or blankets for added comfort during the concerts. Call 838-4343; free. * The folks in Rockville are observing Memorial Day with a ceremony at 9:30 and a parade at 10:30 starting from Courthouse Square. The entourage will proceed south on North Washington Street from Martins Lane to West Montgomery Avenue. The reviewing stand is at Courthouse Square Park. Bring the family and friends out to see at least 70 units in the parade: from majorettes to floats to antique cars. Cyclists interested in riding in the decorated-bike portion of the parade should assemble at Martins Lane at 10:30. Call 424-3738. * Ending the day is the National Symphony Orchestra's annual Memorial Day outdoor concert on the west lawn of the Capitol at 8. Conductor Morton Gould and the orchestra play selections of American music for you to relax to and enjoy. Rain date is Tuesday. Call 426-6700; free.