i like to run. simple. no gym membership. no complicated/expensive gear -- only necessary investment is a pair of running shoes (runs around $90). best of all, you can exhaust yourself quickly: if you don't have much time to spare, just run faster* for less time.
* i wouldn't recommend taking this to an extreme. i knew a guy in college who'd max out the treadmill for a few minutes, sweat like crazy and call it a day. odds are that's terrible for you. (he also had an anger management problem...related?)