Handstand pushups (against a wall) are an *excellent* strength builder. It requires a lot of stabilization and balance and is not easy at first, but it is great for building some real strength. If it's too hard to do or you don't feel comfortable with it, skip it.
Wide and close grip pushups are good for working the chest and triceps, respectively.
Dips, lunges, bodyweight squats, pullups (if possible). Curls with anything handy for biceps. Sometimes you just gotta be creative.