Hey, welcome to my first Instructable! This project is my first toy hack since I was a little kid melting army men together to form, well, melted army men. My kid picks up toys from school "treasure boxes" and restaurants and dollar stores like it is his job. Sometimes he gets doubles. This particular fella was lacking a missile to launch, but the opening looked like a good spot for an LED...
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.

Step 1: Get Toy. Go Make a Triangular-headed Screwdriver. Disassemble Toy.

Title says it all. I found two of these guys in my kid's junk box and asked Jr if he'd mind if we improved it a bit. Gathered tools and found, yes, those darn triangular-headed screws. Read another Instructable about grinding a nail into the appropriate shape (this one), managed to crank one out with a Dremel and also managed to not have Jr grind his own knuckles off, and got to work on the robot. Looks like a prison shiv, works like a dream.

Step 2: Survey Your Domain.

Cracking this guy open I was excited to see a small spring loaded button that deploys the missile, if he had one. The small button switch I found in the aforementioned toy gun would JUST fit under it with a little bit of work. The chassis is also not filled with extraneous support posts or plastic bracing, so I had lots of options for where I could pack the wiring, batteries, etc. The LED, resistor, and wiring fit perfectly along the length of the arm, too, so long as I kept the joins nice and neat. Just had to pull out that longer spring.
Play around with your components, wiring, and the like before you fire up the soldering iron and glue gun. Oh, and put the legs, other arm, and the ridiculously tiny head in a bag for later. Must've misplaced his head 64 times before this simple project was completed.

Step 3: Breadboards Are Fun.

I started to explain the point of breadboards to Jr, but he figured it out pretty quickly: you don't want to find out that LED you poached from that garage sale toy is, in fact, dead AFTER you've put the whole project back together. We played with a few LEDs we pulled from the gun, made static-y sounds with a small speaker it also had, and made a motor twirl. Parts for future projects! Then we got back to our gutted robot after confirming that we did indeed have a working LED (in red, no less!) and the button still worked.

Step 4: Start Assembly.

LED was secured with a little hot glue, then the wiring was fitted along the arm. The preexisting plunger had a perfect gap to allow the wires through without pinching them against the frame! I had to gently remove some plastic below the plunger so the button switch would fit. It's a bit wobbly in there but should hold for at least the length of the life of the little 3v button battery I'm using.
Sorry there's no wiring diagram. This one is pretty basic. Connect positive end of battery (or battery housing if you are lucky in your scavenging) to the switch, continue with another wire on the neighboring switch post to the LED. I connected the resistor directly to the cathode. From the other end of the resistor we carried the current back to the negative end of the battery. Circuit complete. After giving the plunger a test or two we decided it was sturdy enough to commit to closing it up!

Step 5: Enjoy Your Hacked Toy.

Warn the kids not to shoot it directly into anyone's eye. For the love of Mike.
I hope you enjoyed this short Instructable. I look forward to any comments or questions you may have.