I practice chords and tunes from fakebooks on piano because it helps my theory and my arranging/composing. It is such a great instrument to know. I have learned the basics of sax and clarinet because of school, and Tenor sax is pretty fun to play tunes on. I repair flutes for a living right now, so I found a fantastic Yamaha student flute on craigslist for $150 and have been teaching myself flute. This not only affect my repair ability because I can now properly play test them, but it is a fun instrument to play. All of the instruments I try to learn on my own I do so by playing jazz tunes. It gets me familiar with hearing it, reading music from fakebooks and learning scales.