We liked computer software from The Critical Thinking Company- maybe Math Detective or one of the logic games like Mindbenders. My kids also really enjoyed Beestar for several years. I don't know why, but it didn't really feel like homework to them. The gifted math is really interesting.
These kinds of things don't necessarily correlate directly to classroom math - not like a bridge summer book - but I've found that the kids enjoyed them and their foundation for math and critical thinking really strengthened.
Critical Thinking Company Beestar
