Make a prison tattoo gun which can engrave into various soft materials, but you'll need a jack hammer or something a little more powerful to engrave into hard things like steel or a safe.
Materials: pen tube, copper wire, metal bar, batteries, motor, tap, button or wooden square, drill.
1. Take the metal bar and bend it back to form a right angle.
2. Take the pen apart leaving the pen tube.
3. Tape the pen tube onto the metal bar.
4. Tape the motor onto the top end of the metal bar.
5. Tape the batteries behind the motor. If your motor is the same one that I got, then when you have the motor attached to one battery it will give you power level one and when attached to two batteries you get power level 2. Different motors have different characteristics, so you may want to get a more powerful motor or something like that.
6. Take a button or cut out a small wooden square if you don't have a button.
7. Drill a small hole the same diameter as the motor shaft in the center of the board.
8. Drill another hole the same diameter as you copper wire near the center hole in the board.
9. Fit the wooden board or button onto the motor shaft, gluing it in place making sure that is is perpendicular to the motor shaft.
10. Fit the copper wire through the hole in the wooden shaft and through the pen tube.
11. Connect the motor wires to the batteries and see how it works.
12. It likely won't work very well as there are a lot of areas for the copper wire to hit into creating friction. You have to get it set up just right to get the prison tattoo gun to work smoothly. It took me about five hours to get it set up right. So don't feel bad if it takes you that long to get it right.
13. Once you get it right you can engrave on some material like styrafoam and make designs like spirals, horses, knights in shining armor.
I made a small addition to this by 'gluing' two books together. Take two books overlap one page after another page after another page, etc. You will not be able to pull them apart. Mythbusters took two phonebooks and it required two tanks to pull them apart.