Yes, the Bletchley stuff is fun--Harpo spent a lot of time on those lessons this year (they're printable, by the way, Kriston, so if you wanted to, you could print some off to use on days when you didn't have Internet access). Budding cryptanalysts might also like Simon Singh's "The Code Book," which is an excellent read, and has some codes to crack at the back of the book.

peace
minnie