Since I just mentioned the wonderful, inspiring SF Bay Area font foundry, Emigre Fonts, I should share a link to their geometric wonderland known as Crackly. Not only is it very visually satisfying, but there is “coloring book” PDF you can download, to appreciate it FULLY.
The dedicated Crackly page on the Emigre site is here, and you should have a look at it. Go! Do it! It pleases me on multiple levels, as a fan of geometry, fonts, design, my beloved Bay Area, Zuzana Licko’s work, and the playfulness of coloring books.