The gym is always a good place to start. Its proven that working out makes people feel better.

Not sure if this counts as an "activity", but...cry and drink a glass of Sunny D. Seriously, I enjoy a good cry every now and then. It's amazing just how much crying with all your heart when you need to can get that horribleness out of your system. Like, if you're already feeling sorry for yourself, don't stop halfway! And then, when you know it's time to clean yourself up, down that Sunny D. It saps the tears right out of you and puts a pep in your step, to boot!

Now, if by "down" you mean frustrated and boiling over, then: Play video games. The more button-mashing involved, the better for this situatuon. ...Of course though, I think that playing video games is a valid answer to any question in life, so the usefulness of this response may vary from person to person, haha. :)

Visit family. They always get my mood up.

Drink, listen to music, and sing. I also do that when I am happy, or restless, or have a burning case of hemorrhoids, or tired, or ... well it is very hard to tell when I am feeling down.