While guiding my students through the density playground, I started to explore the vast resources of the PHET website, and came across this interesting simulation on radioactive dating:

http://phet.colorado.edu/en/simulation/radioactive-dating-game

Unfortunately, this simulation uses Java, for which my poor out of date classroom computers needed to download an update.  The first class I tried this with was only partially successful in getting it to work on their computers, so I decided to use my teacher computer on the projector to run the simulation and ask students for their suggestions an predictions as we ran the simulations.  I found it not as useful as the Buoyancy Playground, but it is much better at visualizing radioactive decay that previous methods that I have used to help students understand the concept.