“Arrange the vertices such that no edges overlap”
Planarity (which used to be hosted at planarity.net) is a game that is simple in concept, but will have your visual processing areas tying themselves in knots. All you have to do is rearrange a bunch of vertices so that none overlap (creating a planar graph). It starts off simple but gets complex very quickly.
Edit: Duh. Forgot to include a link in the original post.
Much later (April 2022) edit: June Schwartz kindly emailed to let me know that the link points to a Flash version of the website; and here in the future we no longer have such antiquated tools. (My words) They provided an updated URL too, so 10 internet points to them, while I reminisce by having another go at Planarity!
Wasn’t too many levels before I was hopelessly tangled…