I completely agree with Jamie. When I feel like I am doing something good for someone else...I almost always walk away feeling better than before. Without even meaning to! That's one of the added benefits of doing something good!
Bike rides are great for me too - I put on my iPod and bike the prettiest place I can find... being surrounded by beauty (that wasn't created by man out of steel!) clears my head like nothing else. I only wish I lived in Colorado! ;o)
But the most important of all for me...and this may sound silly...but I'm a firm believer in breathing (heehee). A minutes worth of slow and deep breathing can calm me down and cheer me up every single time.