The cool thing about the Wii is the motion sensing. It works because of the sensor bar that sits on top of the TV. But because it's on top, it may fall off and break. We can show you a lot of easy and cheap ways to replace it in this Insider Secret.
The sensor bar is plugged into the Wii, but that's just for power. It's lighting up infrared LEDs. Those are picked up by the Wiimote, and it triangulates off them to determine its position, and then the Wiimote sends that information back to the Wii. So if you have two infrared lights and an independent power source, you can easily replace the sensor bar should it break.
One way is a remote control. Most consumer electronics remotes use infrared light to send information to the device they control. If you have two remotes you don't use anymore, you can tape down the power button and space them out. Or get more ambitious and cut one open and wire the LEDs directly to the power pack, like these folks did.
Another way is to get two LED bulbs, obtained rather cheaply, and two 3-volt lithium batteries. Then place them a sensor bar's width apart. The downside to both these solutions is the work involved and the fact that the batteries may wear out or even burn out the bulb.
So here's my favorite solution: tea lights. Just light the candles and space them out. The infrared light from the candles works just like an LED. Makes for a romantic evening of Mario Kart.