"We run, not because we think it is doing us good, but because we enjoy it and cannot help ourselves. The more restricted our society and work become, the more necessary it will be to find some outlet for this craving for freedom. No one can say, 'You must not run faster than this, or jump higher than that.' The human spirit is indomitable."
-Sir Roger Bannister
If you're stuck for an idea, or you simply need to switch off for a bit, I suggest going for a run. The longer, the better.
Nothing helps me focus more.