Warning: You should always mimimize exposure to radioactive samples by keeping as far from them as possible and having them around for as small a time as possible.
Equipment:
Theory: Alpha particles can be stopped by just a few cm of air, or by a sheet of paper. Beta particles are a little more penetrating, and gamma rays even more so.
Description: Make sure the speaker on the counter is turned on. The smoke detector has an americium source, which is an alpha emitter. These particles are easily stopped by a sheet of paper, or by a few cm of air. The orange feista-ware bowl, however, is coated with (I think) a glaze that contains uruanium salts; the bowl emits both beta and gamma particles, as the count does not drop off completely with the aluminum used as a shield.
It's important to emphasize that although radioactivity can be quite dangerous, and exposure should be minimized, we are surrounded by safe levels of low-level radioactivity on a daily basis. In other words, the students shouldn't panic and go home and throw out their smoke detectors.
Setup time: 24 hours