Best to check with your doctor, but I'm guessing that excessive walking or elliptical is probably not a good thing, especially during the first few weeks.
Here is what I did :
Rowing machine: only use your healthy foot to push, rest the other one on the floor. After getting used to rowing, you get a pretty good workout from it.
One-legged cycling: my gym had stationary bikes with pedals that my mountain bike shoes could clip into. Broken foot went into the water bottle holder while healthy foot pedaled. Couldn't go for very long at the beginning, but once my muscles got used to it, it was great. Also, it was a good exercise for after the boot came off, I could let healthy foot do most the work, while my weak leg got used to exercise again.
Hand cycle machine at the gym: didn't like it but it's an option.
Core exercises, Pilates: did a lot of that, you can do 95% of the exercises in a Pilates class, or just do core work by yourself.
Swimming: my doc allowed it after 4 weeks. Be careful when using crutches near the pool, they don't handle wet floor very well.