Walking/hiking
Running
Cycling (if you have a bike)
Push-ups
Ab crunches
Resistance exercises with elastic bands, which are inexpensive and easy to obtain at sporting-goods stores
Weight-lifting with canned goods or heavy books or low-cost dumbbells from a sporting-good store

The possibilities are endless. Research exercising at home on the Web to find more ideas and information about proper positions and individual exercises. You'll find lots of suggestions!

Edit: I knew THCBongman would come through on this thread. He's an exercise buff, too. Listen to THCBongman. He's a wise soul!