I went to a physio class after my c/s and she said the best way to start on the tummy muscles is core excercises, but not strenuous ones.
Fast, jerky motions are out for a few weeks (obviously, not like I could do any of those anyway).
If you have a diastisis ( i did) try to avoid twisting motions that can pull the muscles even further apart. Don't try situps if you have a diastisis, they do more harm than good.

Other than that I think you can do anything you feel up to