1. Start with a sufficiently amazing secret lair entrance.
Turning any regular doorknob into a secret candlestick switch is pretty easy. However, completing the accompanying motorized secret door is a weekend project for sure!
2. Continue the "hidden" theme with a book light switch.
3. Build a positive reinforcement secret knock training device.
4. Make some robot minions while you wait for flesh and blood human ones.
If you have the parts, make an adorable legion of bumbling walker droids. For a more dapper look, take a Mr. Potato Hat and combine with paper clips and a camera.