I think your problem is time. It will depend on your resources, how many computers and how many printers and what software you were intending to use. Minecraft is of course a good bet for interest but I do not know the sex ratio of users.

There are some interesting tools out there. For 3D modeling there is an online sculpting portal or though we have never tried to produce a model from it.

One thing that we found was 3D printed CO2 cars but this may be slightly advanced. The main thing I would think is for them to see the printer in action so that they can see how it is building up a model and to give them the opportunity to work with a simple 3D modelling suite.