My kids (1 and 3) just love to help me exercise. "Bench press" the 1 year old while "kicking" my legs with the 3 year old hanging on. Be sure to turn them upside down and back. Be sure they "hit" each other. It is not a bad exercise and combines play time.

I've found that a double jogging stroller is a great investment (look for them used), go for a walk, then the kid is tired they get in and then the walk can speed up. Sometimes the kids are asleep when I get back.