I've considered doing this for a while now, but have never had the time/budget to really get down to it. Some of you may, however, benefit from this information.
If you're any good with wiring or soldering, you might be interested in picking up a few small prototype boards such as an Arduino, and a few simple sensors to hook it up to. You can get light sensors, motion sensors, pressure sensors; pretty much any kind of sensor you can find or buy at a radio shack or hobby shop. The smallest and cheapest ones cost about ten dollars and are finger sized. They can handle a number of different power sources, and aside from the wiring, all you need is a very basic understanding of the language it's coded in, which is very easy to learn, and very simple to code, a computer (I'd be really surprised if you could read this without one) and a cable for plugging it in to that computer. depending on the type of board, you may also need an adapter piece for particularly small ones.
you can pick up the smallest size for about 10 bucks here:
https://www.sparkfun.com/categories/103
If you want to get really crazy with it, they make all kinds of radio receiver/Internet send and receive/ GPS Locating Addons. Please note however, that these addons are often intended for full size boards, and can add up rather quickly.
Since it's basically a platform rather than a prepackaged system, you can go as far as you want with it or not. I've seen people doing simple things like getting a speaker to play a song, and I've seen people hooking up "nerf guns" to thermal cameras and motion sensors.