I used to (and need to get back to - been slacking off for a month or so now) do burpees. They are relatively quick, with a mix of cardio and some strength training, and hardly take any time at all.
I usually do the "pyramid" that is start by doing 11, then a minute break, followed by 10 all the way down to 1. It takes me roughly 15 minutes, but I end up feeling really good after that.
Also, taking a break from arduous mental activity is a good thing - watch movies, read fiction, step outside with a point and shoot camera and take pictures. Anything that let's your mind relax for a bit.
Finally, I have found that meditation really helps. It helps to slow you down, improves focus, and the world really seems different once you get used to it. Just concentrate on your breath, for 15 minutes a day (or twice a day) and everything seems a tad fresher and clearer.
Of course, this article (and the comments) have got me thinking about a vacation. I don't take enough time off and I should.