Go into your room and rearrange stuff, maybe clean a bit. When I feel mopey I notice that crap tends to accumulate on the floor of my room. So I what I do is spend five, ten minutes putting clothes back into the dresser, throwing trash away, maybe decide that I don't really need this book series anymore, dust a bit, that kind of stuff. By getting rid of some clutter it kind of feels like I'm de-cluttering myself, it's also something productive to do that doesn't require much thought. Hope that helps!
I second this. Cleaning and exercise help me to do something constructive. Plus it just feels so good to have a clean room.