S.J.
What about doing any type of work out video tape? When I was on maternity leave I did step arobics and my daughter found it pretty funny that I was bouncing around. You could also do simple things with your son...you can lay on your back and put him on your legs and kind of do leg curls, good work for you, fun for him. I would also lay on my back and pretty much use my daughter as a bench press, they think it's fun, and after a few sets of them your arms get a good work out. You can also put him in a snuggly or something like that and just clean and do things around the house with him on you. It doesn't seem like a lot, but it's added weight you're carrying around so it helps. Good luck!