Chess Set Design
When introducing my students to 3D design I like to group them into teams and have them build a collaborative chess set. Usually the numbers work out such that each student can pick a piece (King, Queen, Rook, Pawn, etc) to design. The class as a whole gets to decide the theme of the chess set and once they have to design their pieces and print them on the 3D printer. My students for the most part use TinkerCad with a few opting to use Sketchup or Blender.