I'm getting surgery on my foot in 3 weeks, I'll be in a foot cast type thing for four to 6 weeks, how do I get excircise while I'm off my foot?

You could try doing sit-ups or crunches, or you could lift small arm weights? I know it's not much but it's better than nothing.

Pull ups , Sit Ups, and light weight training depending on your age.

If you get crutches that'll be a workout in itself!