I have developed an actual exercise habit purely because free classes are running at the university and they start an hour after I finish work. I can usually persuade myself to stick around for that hour - especially because it means I finish work a little bit late and then have the "well, now it's X past the hour, and the traffic home will be horrible so I might as well stay" conversation with myself. I don't know what I'm going to do when they end in September.