Remember that scene in Karate Kid ... ok, I just made myself feel old... Anyway, where Daniel-san is balancing on the fencepost or pier tie or whatever that is, practicing his crane no-can-defense kick? That inspired me to try a variation on calf raises. Standing away from a wall so you're forced to balance entirely by muscle action, bend slightly at the knee and lean forward just enough to center your weight in the ball of the "active" foot. Holding that balance there, raise the other foot, and bend that knee slightly too, to cross that leg in front of the active leg. (This helps keep the active knee aligned with the foot.)
Now try to do calf raises. Yeah, thought so.

I just can't raise up on my toes that way, the way I would in a both-feet stance or with the wall to balance against on one foot. Trades off range of motion to challenge the muscles involved with balance, I suppose.