Here is an under desk foot rest that doubles as a balance board. The Yes4All Rocker Balance Board has max weight capacity of 300lbs. It has an anti-slip surface to keep you comfortable. Its wheels add instability to your plank, push up, mountain climbers, and squats.
The Yes4All Wobble Port weighs only 3lbs, so you can move it around easily. You can find out more about it here.