A few years ago my sister-in-law bought a house. She gave us a tour, and when we got to the basement I noticed a neat old speaker hanging from the wall that the previous owner had left behind. She let me have it and I threw together a fun little project. I had an old Airport Express that I wasn't using anymore and I thought I'd make an Airport Express speaker.
Now keep in mind that this was several years ago and I was really just dipping my toes into the whole DIY thing. There are a few things that I would do differently now. In fact, I plan to do a revision sometime soon. The upside, though, is that you can't really get much easier than this.
Step 1: What You'll Need
First, you'll need a speaker of some sort, obviously. This one says "High Fidelity", so you know it's freaking amazing. You'll need something to open it up with; probably a screwdriver. You'll probably want to use a soldering iron for some of the connections, though I did not (we'll get to that). If the speaker cabinet only has one speaker in it then you'll want a 3.5mm stereo-to-mono adapter and an audio cable with one end that's got a 3.5mm male plug. If you've got two speakers you'll want to preserve the stereo situation and run an extra cable.
You'll need a power cable to plug into the Airport Express. If you've got a newer model than you've already got one.
Finally, you need an Airport Express. You have other options as well, which I'll cover near the end.