what kind of music / art do you enjoy? creation is a pretty good way to challenge yourself, and i don't think its the sort of thing you can every really "master."
volunteer work would be pretty cool (and free advertising
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my guess is that, of course, only you can answer this question - but you've got have a couple of things you've thought about and said "that would be cool as all get out to do / be / try..." well, you don't have to worry about failure because you're trying something new to see if you like it. just experiment and see what sticks.