Walking, walking walking. Absolutely the best free exercise there is.
Check out fitness books from the library. There are plenty of ways to use your own body weight as resistance. Yoga books are great to have as well; the best tell you what the pose is supposed to feel like when done properly.
Use soup cans (or baked beans, even heavier) for upper body weights.
Then there's always "housework, fitness style": Squat, bend, run up and down the steps, reach, twist, lug, pull, push ...
The less efficient you are -- "oops! I forgot to run this upstairs on my last trip ... oh well ..." -- the better the workout.