A Klein bottle ( Wikipedia article ) and, in combination, a beer opener.
Or such a thing:
Like? Buy here: www.bathsheba.com/math . There are many interesting gizmos. Only expensive. Hundreds of dollars.
You can also read how such things are done: bathsheba.com/sculpt/process
And you can also try to win such a figurine at the competitions for programmers Al Zimmermann's Programming Contests . Today, a contest called “Son Of Darts” started there. There are no restrictions on the programming language for solving the problem. Actually, you only need to send an answer that you can at least guess.
Only the contest lasts exactly a year.