Heel Rises (or Toe Rises, depending on who you ask)
Begin by standing on a solid elevated surface (such as a low beam or a block), facing a wall. With your feet together, standing only on the balls of your feet, go up as high as you can on your toes, drop so that you flex your feet as much as possible, then push back up to high toes. You should try to do them slowly and with as much of a height range as possible. Do in sets of 40 or more.
There are several variations that target different muscles. Try them on one foot at a time, with your feet turned in, or with your feet turned out.
If you want to make this exercise more difficult, have someone of equal or less weight sit on your back in a piggy-back position. You could also try not straightening your legs completely.
Leg Lift Pulses
Tuck Jumps on a Mat