Someone suggested Chopin Etude Opus 10/1. I agree that, properly approached, this is an incredibly rewarding study piece. But it's only fair to add that you need to be very careful not to make the mistake of letting muscular tension build up. Up until a month ago, I used to enjoy playing it 5 times a day. Then, as I ratcheted it up the final step to tempo, I got carried away with the rotations and let tension build up in the shoulder without proper release. The result: instant tendinitus. I have had to stop playing Chopin completely. There is only one bright side to this experience: The pain has forced me to slow down and play only Debussy and Bach for now, but now with 10x the awareness of what my body is doing at all times, and slowly rebuild the technique. Ironically, it was only after this happened that I got Barbara Lister-Sink's dvd and learned what I had done wrong. So be forewarned, people!