Inspired by the Teenage Engineering Grip Car, this design was intended to be a McMaster Car, one that could simply be 3D printed and then assembled using some basic fasteners from a hardware store. It's been through quite a few iterations to make it strong enough for kids to play with and it makes for a fun project to assemble yourself. 

This assembly uses a few parts that are not 3D printed:

4 x Ball Bearings (~$1) alt
8 x O-Rings (~$3)
6 x M3 Screws ($1)