I will second going outside. Run, walk, stroll, whatever works (probably not driving - though if it's a nice scenic country road, well maybe.). Get your body moving, get some sunlight and fresh air. Seriously it's magic.
Lifting heavy weights works too - I got into it several months ago and it has done wonders for my attitude, energy levels, and overall well being. You don't have to pay for a gym... fill up some milk jugs with sand, hook 5 gallon buckets full of rocks over a broomstick, heave cinder blocks over your head, etc. If the weights are heavy enough and the exercise is difficult enough, you have to force yourself to focus deeply and entirely on the effort, which tends to push out all the other BS that has been bugging you all day.