One other fun math resource my boys have liked is the Murderous Maths series, by Kjartan Poskitt. They're published by Scholastic UK, and are most easily obtainable in the US from Ray at horriblebooks.com. I think FUNbooks in Vermont also carries them (at not so much of a discount as Ray, I believe), and I know some people order them directly from the UK from Book Depository. These might be just the ticket for summer.

peace
minnie