C really loved the "Top Secret Hidden Messages" activity book from Make Believe Ideas.
It's a neat little journal with everything you'll need to become a super-cryptographer, spy, or just a kid who likes writing "Daddy pongs!" in invisible ink (which you can, of course, thanks to this journal's rather neat invisible ink pen with UV light!)
Just in case you think your parents or teacher might have their own UV pen, you can always lock your journal up - yep there's even a padlock on it to keep those innermost secrets safe from prying eyes (though I'm pretty sure Mummy is adept at picking these locks with a hairpin, it's the only reason I can think of that our secret chocolate stash always seems to be empty despite being under lock and key).
"Top Secret Hidden Messages" is available now from Make Believe Ideas (kindly supplied for review).