This project is good for practicing soldering in tight places, basic wiring, and liberating pieces from other broken toys. I would estimate this whole project was free, if we ignore the initial costs of whatever the toys, soldering iron, glue gun, and the like originally set me back. Shall we proceed?
You will need:
1. A suitable toy needing an upgrade. Should be hollow (many fast food toys seem to be) to hold new electric guts.
2. LED of your choosing. I went with a small red one I got out of a broken "Star Wars" style generic toy gun.
3. A resistor of suitable rating to avoid burning out the LED if you think you might like several in series. Calculate using Ohm's law (R = (Vs - Vf)/I, where Vs is the voltage coming in, Vf is the forward voltage of your LED, and I is 0.015 mA. The one I built is pretty mickey mouse and probably doesn't need the resistor I put in there, but it was part of the original toy and I figured it isn't harming anything by being in there. Made me practice some extra soldering at least!
4. A small button switch. You can use any sort of toggle or switch - repurpose something from another broken toy if possible.
5. A 3v button battery. As I got mine out of an old motherboard (the things people throw away!) it had a handy mounting bracket with it.
6. Tools: soldering iron, glue gun, wire strippers, wire cutters, screwdrivers to crack into toy.