Yah, I like both of them a lot, but I find them hard to explain to those who aren't already in some sense "into math." I was going to include Euler's equation, but left it out for that reason. I do sometimes explain 3-coloring of graphs, and can sometimes get to NP-Completeness, although it's a bit of an effort.

However, I don't want to preclude stuff that maybe can be explained, even if at first glance it doesn't seem so.