Sometimes just playing melodies from songs that you know. Whatever that are, is fun. Christmas carols, kids songs... old stuff.
If you read music, you'll get a world of enjoyment and music experience from digging into the Cello Suites of J.S. Bach. Something else that is fun for reading and for expanding musical horizons is to get some orchestral bass parts (check out
www.shar.com) and play along to the recordings. Start off with Haydn, Schubert, Beethoven and work into the more demanding composers. Yea, I know, you won't ever perform this music on BG, but its interesting to try different things.
Just jamming to the radio is highly informative, especially the commericals.