I like listening to things like Bach's Concerto Number 5, Fantasy and Fugue and Partita Number 1 while in another state of mind, although they're played on the harpsichord rather then piano, which is a good thing really...
Chopin is also rather relaxing, you could also try Liszt Liebestraum Number 2 or as said: Beethoven Sonata Number 14 - the Moonlight Sonata.