The "chair" that I've most recently begun using several times a week is
an exercise ball (inset picture) which I sit on in front of my desk while using the computer, and
I use it as a footstool while sitting in the arm chair to exercise my legs;
supposed to build strength and balance.

(The two fellows in the photographs are 2 of my brothers ~ the one on the left was killed by a drunk driver the year after that picture was taken.)