DoctorWinstonOBoogie

I suggest the incredible Curta Mechanical Calculator. It is an incredible calculator that is totally mechanical. It was created by an engineer named Curt Herzstark in 1948. It is somewhat rare, and therefore a bit expensive. It would be really cool to have one (or at least a replica). I cannot do it justice with a block of text, so I'll stop here.

I don't really have any specs or anything for it, but these resources (especially the Curta Calculator Page) should have ore than enough. I think this would be a great project, and a challenging one to test your abilities in 3D printing! All the best!

Here are some great resources:

Numberphile

Wikipedia

The Curta Calculator Page

Simulator

If you had not quit high school, what do you think you would have done in life?