One awesome idea I came up with for the playfield was a target that could move around. This target became the “Super Agent” that tries to stop your world domination plans. I took apart an old printer, stripped the belt and motor out and rigged it up so a target attached to the belt moved across the playfield.
The whole thing is controlled using an ATMega8 and a L293D H-Bridge chip for the motor. There is a switch at either end of the belt to trigger a change of direction. The target itself is a switch, consisting of two metal rectangles with a small separation. If the target is hit with enough force, the two rectangles make electrical contact which is relayed to the microcontroller through two thin copper wires. This may sound fragile, but it works surprisingly well. Check out the video to see it in action.