I've had a very good experience working with a 10lb. sledgehammer for 15 minutes at a time.
Sledgehammer work is very versatile, and you can work every part of your upper body by doing a variety of movements. Your functional fitness improves remarkably quickly, and it's surprisingly fun and satisfying to throw a sledge around for a while. It also doesn't take up much space: I work out in my tiny New York apartment without a problem. After 3-6 months of working with a sledge, you just move up 2lbs in weight for a new challenge.
You can find more information about sledgehammer workouts
here. Have fun!
Josh Kaufman
Josh Kaufman: Inside My Bald Head The Personal MBA — Mastering Business Without Spending a Fortune