If you aren't dealing with a big power supply on your board, or super-fast switching times, I'd keep it simple and use a single ground layer to keep everything tight. In between your noisy circuit sections (analog vs. digital signals), put a "wall" of vias, to keep return currents within the specified areas of your board (analog, signal, digital). Be sure to stitch the board together with lots of vias, to keep the ground potentials even across the board (probably not much of an issue, since there is going to be little power draw, given the circuits mentioned).