Animate a paddle-and-ball game on a 128×64 OLED using non-blocking updates and debounced controls.
Keep accurate time with DS3231 and render it on chained MAX7219 matrices using an adequately sized 5 V supply.
Combine buttons and an IR remote to operate a tabletop servo latch as an educational state-machine exercise, not a secure safe.
Switch a 12 V solenoid from a separate supply through a rated driver or relay, with flyback suppression and a fail-state chosen for the mechanism.