An arm ergometer is a great tool for people who can't use their legs. It's basically an exercise bike for your hands.
There's a chair workout video (at least one) on SP. You may have to modify it for your needs but check it out.
There are tons of strength training exercises you can do for your upper body and core that don't involve legs at all. Just look at all those bodybuilder dudes who never work their legs! ;) If you have access to a gym or even if you don't, there are literally hundreds of exercises you can do for your arms, chest, back, lower back, shoulders, abs, obliques, forearms, upper back. Most of your muscles are located above the hips.