-calisthenics (push-ups, crunches, and the like)
-walk rather than drive when you can (and save the gas $$ too )
-i've read about some people using like soup cans or bottles of water in place of weights
-dance to a cd you already have or to the radio
-walk around the track at a local high school or middle school
-take your kids to the park and play with them, or just go yourself and join a game of basketball or something
-find out about any nice hiking trails in your area and go on a nice day
-someone on here recommended freecycle for free fitness equipment if you have the room

That's all I can think of for now.