Beer and crisps
I like to invent some new practicals every year but when one works so well its worth doing again, the exponential decay of beer is a winner. There aren't so many experiments you can do with radioactive decay so when you find a good one its worth hanging on to it. I actually still have a set of sources and a protactinium generator but its difficult to do a class practical with such limited resources (probably not allowed either). The bubbles work quite nicely because there are a lot of them and they pop quite randomly. I was planning to make an experiment with some of those plastic hemispheres that you invert on the table and they suddenly pop up but they just weren't random enough so I abandoned the idea. The beer one is fun because it's beer (actually non alcoholic beer). Whenever I buy beer I always have to buy crisps (potato chips to non Brits) these have nothing to do with the experiment but add to the party atmosphere at 8 o'clock in the morning. The results are pretty good especially if you use a video camera giving a half life of around 50s and a much better understanding of exponential decay.