What would you do with an extra room in your house?

My room mate recently moved out, and I'm not planning on getting another one anytime soon. This leaves me with a spare bedroom in my house that I'm free to do what I like with, but I'm not sure what to do with it. A Library/study is a good idea, but I don't really have the carpentry skills to make... show more My room mate recently moved out, and I'm not planning on getting another one anytime soon. This leaves me with a spare bedroom in my house that I'm free to do what I like with, but I'm not sure what to do with it. A Library/study is a good idea, but I don't really have the carpentry skills to make it look great, and professionals are expensive. I'm a single 20-something, so something crazy like a pool table or game room or entertainment room is conceivable, but I dunno if that's the way I want to go.

So, any suggestions for me? And, out of curiosity, what would you yourself do personally (i.e., not necessarily what you'd recommend for me)
Update: Oh, and my building has a gym, so while it might be nice to get some of my own equipment, it's a bit of an unnecessary expense
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