The impossible triangle is probably a great starting point if you’d like to create some illusions of your own. Check out the history and explanations for this special shape. The Hall of Illusions has more great pages like the one on the impossible triangle.
Check out more art by Oscar Reutersvärd, M. C. Escher, Salvador Dalí, and Rob Gonsalves. You may also like this optical illusion post by the Old Wolf. One of my favorites is this animation of the Spinning Dancer.