The project uses an arduino and a beam speed sensor to measure rotational speed. A python program is included to receive the data sent from the arduino on a PC. This is, in my opinion, a nice littel project to get familar with the basics of arduino programming. It features variable declaration, setting up the program, the main loop, an interrupt and serial communication with a PC. It also includes data smoothing via a Kalman filter, so you have all elements usually found in most any arduino application, yet it only is about 50 lines of python and some 80 lines of arduino code.

It fits on an arduino micro and the sensor is cheap so you can have fun for about $ 10,-.