So I found something awesome through my university gym today: it's called the Rec Pack. Basically, to get into the university gym you need to swipe your student card, and then again when you leave. So it records how long you are there for. Now, if you're part of this rec pack thing, it gives you a point for every 1/2 hour of exercise (you can also report your own hours online). There's also other criteria for getting a point (don't eat junk for a week, visit the doctor for your annual check up, etc). After a certain number of points they give you prizes. Tshirts, water bottles, stuff like that.
Apparently, most universities have something similar in place, so maybe check it out?