1. Get up and walk around! This is probably my #1 go to cure. A lap around the office can work wonders, and extra bonus if you make a new friend in the office or combine it with a "work" conversation!
2. Grab a glass of water. It requires me to move away from my desk and then when I sit back down I have something to do (duh, drink the water).
3. Turn on some music! I have an iPod docking station at my desk and I'm lucky enough to typically have my office to myself, so I can play my music as loud as I want. Even when my officemate is here, we have the same taste in music so we have sing alongs! Even if you are working in cramped quarters or in cubes, you can still rock out with your headphones! (Don't forget to be mindful of your coworkers though!)
5. Mentally check out for a minute! Check your Twitter feed, read CNN.com, write an email to a friend, catch up on your favorite blogs...whatever you enjoy, take 5 and do that (as long as company policy allows!)
I wish I could come up with more, but I'm fresh out of ideas. What do you do to rejuvenate in the middle of the afternoon?