This project uses an old RC car that triggers a small gadget that makes funny noises (like farts :P) - but it can trigger almost anything! :)
It can be done in 10-30 minutes, depending on what you've got and your experience.

Parts list:
- An old RC toy like an RC car, the smaller the better.
- The remote control for the RC toy.
- The battery for the RC toy.
- Something that makes noise when powered up or when a button is pressed - mine makes farts! :)
- Some wire
- Something to connect the wires: soldering iron, tape, alligator clips... :)

Step 1: Disassemble Things

Take the toy, unscrew all screws and get to the circuits. There's always a board where an antenna, the battery and the motor(s) are connected. Disconnect the wires - you can cut them, desolder them... Just remember to leave them long on the board. Remove thae board from the toy.
Disassemble also the thing that makes noise - get the board, because it's smaller and easier to connect.

Step 2: The Antenna

The RC toy has an antenna. If it's soldered to the board, perfect. But most antennas are connected by contact with a screw or something like that, and are made of aluminium, so you can't solder them to the board. I just cut a piece of wire to the same length of the original antenna and soldered it - works perfectly. This is where a soldering iron helps a lot, if the antenna isn't already soldered, it's tricky to get it to stick to the board with tape!

Step 3: Let's Make Some Noise :)

Ok now we're going to connect the thing that makes noise. You can have two cases:
1) It makes noise when powered (very rare!)
2) It makes noise when a button is pressed

Mine makes noise when powered. If yours do too, just check if it works at the RC toy voltage - probably it's going to be OK, especially if you don't keep it on for a long time.
Just attach the contacts were it gets power (from the batteries) to the motor wires. When the remote control tells the board to make the motor spin, the board will activate the thing :) Note that those little gadgets work with direct current (like those from batteries) - so, only one of the two buttons (forward/backwards) will activate it, because of the polarity.

If it makes noise when a button is pressed, you need a relay. A relay is like a button, but it is triggered by an electric current instead of a press.
Most of the time a 5V relay will be OK, check the toy voltage to be sure.
Most of the time buttons are NO (Normally Open), so you need an NO relay (almost every relay provides both a NC and a NO switch - so you don't have to worry about it).
Desolder the button and attach two wires from the relay NO contacts to where the button was connected.
Then attach the wires that were connected to the motor to the relay coil. When the remote control tells the board to make the motor spin, the relay will be activated and it will emulate the button press.

With both metods, the thing will be powered/the button will be pressed as long as you keep the button on the RC pressed.

Step 4: Test It

Ok, connect the battery to the toy and press the buttons that control the motor on the remote control. If it doesn't work, check the wirings. If it does, shout "Yahooooooo!" and find a victim :)

Step 5: Putting It in a Box

It could be used without a box, but with it looks way more cool and it's easier to hide.
I used a box bought from Futurlec some time ago. It took some time to fit it in because the box is small.
And this is the finished thing :) Good April Fool's day!