Mountain biking. I ride alone more often than not, and basically always with earbuds in my ears, playing music. Not uncontroversial in the MTB community, but this is the way I like to experience it. To me, nothing beats being in the woods, on a bike, rocking out to some Slayer, Testament, Metallica, Iced Earth, etc.<p>Note, of course, that this is actually <i>tiring</i> from a physical perspective, in the short-term. But it's how I stay sane and it helps me clear my mind of all the random crap that builds up during the week from work and what-not. And over the long-term it is beneficial physically in terms of helping improve overall cardiovascular conditioning.