I would like to propose you a few websites with fun stuff to print using a 3D-printer.
To be updated…
https://www.compliantmechanisms.byu.edu/maker-resources
This site proposes “compliants” objects. They are flexible objects but very effective: this is, for example, a compliant mechanism that prevents a nuclear bomb to goes off unexpectively. There are also used in medical robotics.
https://www.thingiverse.com/
Of cours, thingiverse is the library to know when you are searching for a 3D object to print.