Run intervals. I work out in my basement, and that's what I mostly do. Mine are very simple, more of fartlek: I sprint to the end of one block, jog to the next, repeat anywhere from 10-15 times. (It takes me 20 minutes to half an hour.) I hate cardio equipment; it makes me feel like a lab rat.
I also garbage-picked a heavy bag, and that's chained to one of the beams in my workout room. I sometimes do boxing intervals when I'm sick of running.