Put your phone on a balance or wobble board, set the stop angle, and Balance Timer will measure how long you can stay upright on the board. The timer starts when the board is straight and stops when it exceeds the stop angle.

Large numbers and choice of fonts and colors for ease of reading while standing up (this uses the same display engine as my fairly popular Giant Stopwatch). Every ten seconds you get a voice announcement of how long you've been standing (you can turn that off, of course).