Not a big fan of caffeine but what i like to do is go for a bit of a work out...nothing too strenuous/light excercise (ie 15-20 minutes-anymore you may feel even more tired)...it could be a run, walk, push ups whatever I feel like and I find that getting some blood pumping to the rest of the body usually combats that fatigued feeling you get after sitting at a desk all day.
My dog perks me up. He's always happy to see me and hang out or play. Watching the fish and some good music helps too. If you are super stressed out, I recommend a glass of wine, or better yet, a good glass of bourbon.