If you want to keep it light and breezy, I would look at using living books. You could look at livingmath.net for ideas. This would allow you to introduce above level concepts without the pressure of output.
Many people enjoy the Sir Cumfrence books and Theonni Pappas's books. We preferred the more advanced "Young Math" books, bases, binary, venn diagrams, etc, and "The Number Devil". Either way, I would try to get them from the library or ILL since many of these resources are both out of print and quirky... So it is hard to predict what you and your kid will like.
