There are a number of pieces which bring on a smile. In piano, I could mention the scherzo from Beethoven's Op 31/3, Liszt's Grand Galop, Debussy's Golliwog's Cakewalk, the finale of Prokofiev's 4th sonata, Moszkowski's delicious piano concerto.
Non-piano, there's always the last movement of Schubert's Octet, Sullivan's Overture di Ballo, 'Joie du sang des étoiles' from Messiaen's Turangalila, I'm sure I'll think of many more.
Elgar's Serenade certainly wasn't meant to be any exercise in humour, but the sheer mastery of the writing, his sense of atmosphere, that peculiarly Elgarian pathos and yearning, yes, a smile though a few tears.