Cut-the-knot ...
http://www.cut-the-knot.org/index.shtml... is worth looking at. It has a lot of interesting puzzles.
I find many of the questions easy to understand, but the answers very difficult to find. Even when the answers are explained, there are a lot of them I don't find easy to understand.
A small number of the puzzles I remember from school (e.g., Towers of Hanoi).