I'm finding that taking a song I know well enough that I know exactly what I mean to play (even if I don't always execute it perfectly) and playing it in a completely different style is a good way to stretch myself. A few weeks ago I tried this with Tammuz, which I had pretty well set as a performance piece with a flatpick strum. I tried a double time fingerpick pattern, and (1) it pushed me to hit my Bms cleaner and quicker, and (2) I really liked how it sounded and I think I'll be playing it that way in circle now. Tonight I took Harbors, one of the first songs I really worked out something that felt like mine, and played it with a flatpick instead of fingerpicking. I'm not sure I want to make it the new way I play the song, but it made a song I love but hardly ever play feel fresh and new again.