Walking
A month or so ago, I decided that I did not get enough regular exercise, so I started going out for walks regularly. At first, the distances were relatively short, but after a week or so, I decided to stretch them out so that each walk lasts about 2.5 hours and travels somewhere between 3 and 4 miles (I'm estimating on this one; I haven't actually measured the distance).
It's odd how it works - and it comes about really automatically. After a few weeks of doing this regularly ("regularly" meaning somewhere between three and five times a week) your legs get used to it in a very odd way: if you skip enough days in a row between walks, your legs almost beg you to go back out and walk. I'm still not entirely sure how it works; I've heard that the body can come to crave regular exercise, that exercise improves one's psychological state, and there are some theories that exercising may release endorphins, the same as doing something like eating chocolate. (And eating chocolate, of course, kind of cancels out the benefits of the exercise).
I've always been slightly ADHD (I suspect; it has never been diagnosed officially); I've always had a difficult time paying attention to something for more than a specific amount of time and I've always had trouble sitting still for longer than a certain period of time. The more that I walk, the more that this becomes a problem. And as a graduate student, the time that I spend in front of the computer is obviously quite a lot. But after an hour or two of sitting in front of the computer, I simply cannot take it anymore and I have to put on athletic shoes (or Crocs or sandals) and go out and walk a few miles. It's odd how it happens - I used to hike plenty and this never happened to me then.
Believe it or not, though, I find that I actually rather enjoy it.