Exponential decay and half life
Exponential decay will probably be a new idea to students. The experiment and the model of decay introduce them to the constant ratio property. It gives them an opportunity to measure a half-life and build a picture of how it comes about through a fixed chance of a nucleus decaying.
The decay of protactinium also gives students a striking demonstration of one element transmuting into another - changing their previous perception of the unchanging nature of elements.