The Push Up!
Push ups are a wonderful full-body exercise that can be done anywhere by anyone. See here for the basic form. Start with one perfect push up. Gradually add more each day.
*Start on hands and knees and only bend at the arms.
*Bend knees ('girl push ups') instead of extending legs
*Hold at the top in a plank for 10-30sec (great core work)
*Let a kid sit on your back
*Put a non-mobile baby under you...give her kisses or fmake unny faces every time you lower
*Have your kids do them with you