Try this circular balance board AKA a wobble board: http://www.fitter1.com/wobble-board-16.html

I've had two foot surgeries, and to regain mobility and balance this is amazing. There are two: 16" and 20". The 16" one is definitely tough to do! I use it A.M. and P.M. during the two minutes I brush my teeth, lol.

They come with a small hemisphere under the board, with the option of being fixed, or variable. If you make it higher of course, you get a more intense angle, so it's quite a bit harder.

I believe runners often use these single-footedly to build up their calf muscles.