Leg Press "calf raises"... heels hanging off the bottom with the weight on the balls of your feet. You can really load on the weight. I currently do 6 sets of 8 with 710lbs. When I'm on a cycle I'll put the weight up a bit but mostly I'll just raise the reps. Slow and controlled, don't bounce or you'll be out of commission for a while. They WILL grow if you do this with substantial weight.