You want OS9 - give them only a mouse, get into your app (whether PowerPoint, or what - the best option is one with no menubar), and kill every other aspect of the system.
Perhaps even delete the Finder, and put the app used in its place - rename it "Finder", and change its type/creator to FNDR/MACS.
Even rebooting the system will take them back to the app.
HyperCard would be the best option, but it can't do graphics higher than 8-bit
Perhaps HyperStudio or something, but its nothing compared to HC...
PowerPoint is probably your best option. Disable all menu's, and that little box that you can get to in the corner...