One of the crafts I’ve practiced the longest is Origami. I haven’t done much with it in a long time, as I flit from craft to craft like a spastic butterfly at times, but I do dabble with it during the occasional moment of free time when I don’t have much available to entertain me. Since all you need for Origami is paper, it’s an easy craft for spontaneous entertainment.
One of my all-time favorite origami books is Unit Origami: Multidimensional Transformations by Tomoko Fuse. It’s a wonderful collection of origami projects based on combining finite units into potentially infinitely expanding systems. It’s a math geek’s dream book of origami. I’ve read and used this book more than any other origami book I own, and I own more than a few. The draw of Unit Origami is that each individual piece is simple to build, but recombination of those units can lend great complexity to the piece. And certain pieces can be grown almost indefinitely. The strength of the substrate (paper) being the only real limit to how expansive the structure can be, since at some point the structure will collapse under its own weight if built too large.