The current issue of Popular Science contains instructions for a DIY... Bubble chamber!
Those of you who aren't nuclear physicists may not know what a bubble chamber is. In short, it's a way you can "see" subatomic particles - or at least where they've been. I'll summarize the article here, though.
Make a box with at least one clear side or window. We'll be using dry ice, so glass is probably not the best material.
Get some pure alchohol. Very pure. Like 100%. What's in your liquor cabinet is almost certainly not good enough, and if it is, you need a new liver.
Put the box on some dry ice. Make sure the top doesn't get cold, though. The box must be absolutely stable; shifting due to the ice subliming is a bad thing.
Get a chunk of slightly radioactive material from a smoke detector, watch, whatever. Put it in the middle of the box in view of the transparant section.
Soak a small rag in the alcohol, and put it in the box. Close box. Wait and watch.
The idea is that the alcohol evaporates, then supersaturates the air near the bottom of the box, making a thick alcohol fog. The decaying particles from the radioactive sample disturb the fog's local equilibrium enough to leave a visible trail of tiny bubbles. You can do this without the sample and "catch" cosmic rays or background radiation. The sample could also be placed next to the outside of the box.
This is a simplified version of how scientists view the reactions in supercolliders.