I like to literally work out with my kids. As in, I use them as dumbbells and weights for my workouts. Here are some of exercises I like to do:

1) Benchpress - I hold Charlie at his neck and butt, and benchpress him in reps of ten. He LOVES it! I usually do 40 or 50 reps. When I'm done, he's always begging for more.
2) Pushups - I lie down and Charlie climbs up onto my back. Then I do pushups while he holds on for dear life.
3) Curls - I hold Charlie or Olive in my arms and curl them from my lap to my chin. I can't curl Charlie, but Olive is light and small enough that I can curl her.

Anyone else use their children as free weights? <-- See what I did there? Hehe...