Lockpickers, school yourself.
"In terms of neat mechanical things to build out of wood, I figured a single dial sequential combination lock would be a neat thing to make. It would be relatively simple, involve movement, and also show people how a combination lock actually works.
I spent some time thinking about it, and the design I came up with was the simplest that would also be visually appealing that I could come up with. Unlike a real lock, my priority was to show how it actually works."
Once you're familiar with the mechanics of the combo lock, learn how to crack it!
You can buy detailed plans for this model here.