I'm not one of those types to hit the gym; don't think I'll ever be (i.e. even when my kids empty the nest); so, I've found the weilding way: I bicycle to work. Yes, we only have one car, and luckily my job provides me a bus pass; but, I've found that it's worth the extra time to bike in: it's relaxing, I never have to wait, and I'm getting that bit of recommended daily excercise (3 miles/15 minutes one way).
Another key to success is to try to be active: playing with my kids, which [plug alert] includes being my son's soccer coach (i.e. trying to actually run around); see pghdynamo.org; and, using a push mower (you know, spins and chops because you are pushing it) or shoveling snow (which both lower my carbon footprint and initial purchase price--over their motor counterparts), walking the short distances (as opposed to driving) to get the kids to school, grocery shop, pick up a rental movie, or nearby activity (ah, the joys of city living), taking the stairs (not elevator), playing tennis, and biking for fun (not just to commute, but with the family).