Running- Aside from time-efficient health-benefits, nothing quite matches the endorphins.
Swimming- I have a love-hate relationship with it, but it's hard to beat a 30 minute lap sesh.
Weighlifting- it's satisfying and everybody likes gains.
Intramural sports that involve running. IMO, nearly everyone should play pickup soccer, ultimate, or whatever floats their boat once a week.
I don't spend much time learning new songs anymore, but I still love to play:
Maple Leaf Rag by Scott Joplin
Nocturne in Eb Major by Frédéric Chopin
Moonlight Sonata 1st & 2nd movements by Beethoven
Time and Dream is Collapsing by Hans Zimmer
Various jazzy progressions