Examples of cheap sensors: MEMS microphone, camera, voltage sensor, mass sensor, distance sensor, multi-axis position sensors (gyroscope/magnetometers).
Examples of effectors: Any kind of motor, solenoid, LEDs, etc.
If you want to constrain the question to 3D motion, here is a suggestion - hack the controller of any existing RC car platform. Add overhead position sensing within a fixed arena added via external camera. Maybe add a MEMS microphone (USD$3) or position sensor (~USD$10) to verify airtime/orientation. ML problem #1: Add a ramp. Try to get it to jump highest (longest time airborne). ML problem #2: Same with a power efficiency metric. ML problem #3: Same with a time efficiency metric applied to navigation from a random start point and orientation. ML problem #4: Motor noise vs. jump height optimization.