Find some open, outside air and stay there for five minutes (by the watch!). It doesn't matter what you do. If you feel silly just standing somewhere, walk around the block. Stop and tie your shoe. Pretend to windowshop, but pay attention to being outside. Better yet, just let people wonder what you're doing. It doesn't matter if it's hot...or cold...or windy...or raining. Find an awning and stand under it; it's only five minutes.
Then come back. If you're anything like me you'll feel the difference. It's unlikely to give you a spiritual revelation or an insight into enlightenment, but it's almost guaranteed to break you out of your routine, your stresses, the one-track mindset we all get into at points during our day. Even if you are just kicking back and cruising the Web for fun, try it anyway. It can give your brain just a little jolt that helps you perceive whatever you return to doing in a clearer light, in a better perspective, or just with a little more calm.
This is a great one to try to remember: It requires no special equipment or planning. It can be done almost anywhere (unless you're on the space shuttle! ;-). And it can be done almost any time. (It's hard to find a situation in your daily life that you can't excuse yourself from on impulse for five minutes on some pretext or another--think about it.)