Inspiration stuck a couple weeks ago when I was at the local Five Below. They have pool noodles for a dollar apiece. That just invites some sort of project. Here's what I came up with, thanks to some scrap lengths of 2x4 and 1/2 inch PVC pipe. It's part erector set, part Tinkertoy, and part geometry lesson.
I'm still working on the best configuration for the connectors. All the ones I have so far lie in a plane: triangles, squares, pentagons. While effective at making some platonic solids, they're a bit limited. I guess I'm showing my geeky biases here. It's a work in progress.
Construction was as simple as I could make it. With a chop saw, you can convert an eight foot 2x4 into a set of blocks in a couple minutes. Drilling the 3/4 inch holes in the sides takes longer. Alas, I don't have a shopbot handy, or I could have streamlined construction even more. It was still relatively quick work once I put together a jig from the scrap generated by the block production.
To hold the PVC in the blocks, I went with drilling and screwing. It's probably much faster than gluing and it gives the kids the flexibility of removing some of the pipes if their design doesn't use all the possible connections in a given block.