E.P.
Don't get a treadmill - - too many accidents with babies. If they crawl up behind you, their fingers can get caught on the tread and "sucked" down and around into the back slot and mangled. Also, one little 6-year-old friend of a friend's daughter here in NE Texas was on it, fell, got pushed to the back of it, the wall stopped her, and the tread just kept spinning and took OFF all the skin from her forehead!! She's going to require plastic surgery. It was so bad!
I'd recommend a re-bounder, a mini-tramp. Cheap from Walmart. Less shock to the knee joints - - which is a good thing. The pull on the body at the bottom of each bounce strengthens internal organs like the bladder and kidneys. (maybe as good as Kegels?). Just jog-bounce lightly to start. Add scissors, knee-ups, and other fancy footwork later. MOMMY WARNING: Keep the mini-tramp standing up in a closet and away from the kids. One badly-timed, 2-kid double-bounce can send the littlest one flying off head-first into a brick fireplace or glass coffee-table. Or, you could PAD an entire room and put the mini-tramp in there! Eh - - kids will figure out a way to get hurt anyway.