One aerobic exercise you can do (provided you have enough space) is to use a small, portable trampoline. They sell them at WalMart for like $20.
We do this now instead of a gym membership
and find it is a much harder workout than ellipticals, stationary bikes, etc....
The trampolines fold up and can be stowed easily, taking up a very small amount of room when not in use.
If you are doubtful as to how much of a workout you can get from one (as I was), try it out. You'll be at VO2 Max in no time if you jump as high as you can for 10 mins.