At this age, “play” and “exercise” are one and the same, but there are specific things you can do to encourage muscle development. Simply moving the kid’s arms and legs in aerobic motions like hands over the head or toe touching will help with flexibility. To improve arm, shoulder, and back strength, keep toys in a bin that it can’t be knocked over, so the kid has to reach in and pull things out one by one. Encourage pulling up to stand on low objects by placing toys on them. For serious bicep action, try some of these.