Everyone has at least one extra mouse laying around; why not get some use out of it? Disassembling the mouse you can get two optical encoders plus four infrared emitter/detector pairs. Then you can reassemble your new sensors into a quadrature incremental optical encoder. You’ll be able to tell how much the sensor has rotated and by comparing the signals from the two IR pairs you can also determine the direction. These cheap devices would be good for keeping track of robot wheel movements or doing absolute positioning telescopes.