Proximity Keypad

http://www.priority1design.com.au/qkeypad.html

Proximity keypad detects proximity to numbers or regions on a keypad, but do not require physical contact. Ideal for sealed environments, extreme/explosive environments, or hidden behind walls.

So, you could set up a proximity keypad behind the wall, and then on the front of the wall, have a pencil drawing of "my secret keypad", and then say, hey, watch this, punch in numbers in your crude, hand-drawn keypad, and have it activate lights.