Turn Arduino into an adjustable on/off timer set with a potentiometer, with a safety note for controlling mains-voltage loads.
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.
Build local phone control with an HC-05 and isolated relay, with correct 3.3V signal handling, safe startup logic, and mains precautions.