Running/walking with stroller, and later as they ride a bike or scooter, is excellent. Also, invest in some good DVDs (I like Jillian michaels), and start building sets of dumbells from 5 lbs (less if you are brand new to fitness) on up--you can add sets as your strength grows. You can do DVDs at nap time, or perhaps even as they play on the floor beside you.

I find that these type of home, or at least start-at-home workouts (like walking or running) work best for busy moms! Mine are teenagers now, but when they were younger that is how I fit it in. I would bribe them with playing in the park if they would stay in stroller! And then at about age 9 I would walk up and down my street while they played in the yard (I live in a very quiet neighborhood). Eventually I was able to leave them while I ran a little farther from home (like around the block) and now I can leave them at home for a normal run!

Where there is a will, there is a way!

I always enjoyed a living room dance party. 20 minutes of that is as good as any other cardio workout.