Polyominoes and polytans provide natural follow-ups to the tangrams. Let me know whether you have interlocking cubes, (or for that matter any sort of cubes or tiles, or graph paper), and I can suggest some interesting activities.
The best ways to make polyhedra (in my limited experience) is with tabbed polygons, connecting them with rubber bands (this is hard to describe without pictures, but you should be able to find someone who knows how to do it?) or with flexible straw polygons, attached to each other with tape. Both forms discourage perfectionism and encourage exploration.
> The Escher drawings took a *lot* more time than I planned It's OK -- you learned something, and they had fun!