Cook or bake. Make something sweet and chocolatey, or a favorite food. Not only is cooking a pretty relaxing, almost zen exercise, nothing warms the soul quite like home-baked cookies, brownies, pasta, etc. So you'll feel better while cooking, and then you get to cheer yourself up with whatever it is you've just made. And even if you mess something up and wind up burning whatever you made, you've still had an adventure.
Writing is an excellent idea, too. I find old-fashioned pen-and-paper writing to be particularly cathartic. Once I get scribbling, it's hard to stop.
Go out with friends or family, and let them know you love them. Just don't be alone. It makes it worse.