Calves and the sole of the foot
Rock forward for the calves and Achilles. Rock back for the plantar fascia. Change the angle until the stretch matches how tight you feel.
Two minutes beside the desk
Leave it next to the chair. You do not need a mat, a class, or a change of clothes.
Small enough to keep out
A compact board for both feet. Pick a color you will actually leave where you can see it.