One of the first puzzles I remember solving on my own was the one where you draw a house without lifting the pencil from the paper. I think I was 27 years old at the time. (Now that I look at it, I really wonder about this house. What's with the big X spray-painted on the front? Has the house been condemned, and if so, why? It looks like a perfectly good solid house to me.)
In the past, I've used this puzzle to demonstrate polylines. (See Programming Microsoft Windows with C#, page 186), but here's a XAML version:
Did I mention that it's animated?