Replace mechanical buttons with a TTP223 capacitive touch sensor working as a toggle with a relay module, with a safety note for mains-voltage loads.
Wire a GY-521 (MPU-6050) module to Arduino to measure acceleration and rotation and infer tilt angle, with applications in robotics and drones.
Build a robot car that moves and avoids obstacles automatically using a distance sensor rotated by a servo motor and an L293D motor driver shield.
Build a soil moisture measurement system with a NodeMCU board and display the result on an LCD screen, with calibration tips for better accuracy.