There are plenty of bodyweight exercises you can do with great benefit. For the upper body, push-ups are great and hit all the major muscle groups. Look online for various types of push-ups to vary your routine and keep it interesting.
For the legs, you can do hindu squats, front lunges, rear lunges, side lunges one leg squats and more.
In general, do a search for bodyweight exercises and you'll find a host of inventive things to do.