"Japanese Erasers" have been very popular with kids for a several years, as have keychains that hang in bunches on the outside of backpacks. I find both, though the erasers in particular, to be excellent 3D learning tools. When taken apart they're reduced to basic 3D shapes that can be modeled on the computer then reconstructed as 3D printed objects, which are wonderful keepsakes of their introductory modeling class. I also consider joinery to be an important early skill for young inventors to develop. So the most interesting designs in my class are those that consist of multiple pieces that are interesting whether together or as a pile of printed parts. The creation of this simple object, together with others that I've uploaded to Thingiverse, are useful exercises for anybody learning 3D modeling.
my first name at inventorstudios dot com
I've printed this in ABS with 3 skins, 20% fill and .20mm resolution. The cap is printed with supports, the inside with a raft, but the base without either.
I like a tight fit between parts. So the tolerance of this model is .50mm. If you're interested in evolving this, the model source is available on Tinkercad.com.