at the gym I have seen hand cycling machines - your feet dont move and you sit on a chair and cycle with your hands, when I tried it it was very challenging!
anything else that elevates your heart rate such as shadow boxing.
You basically want to keep your heart rate above a certian level. Try circut training with light weights...go from one exercise to the next with no more than 30-60 seconds rest inbetween. That should do the trick at least until your foot heals.
