I agree the Perplexors are great. Be aware the math perplexors are easier than the grid perplexors for the same stated level. So if your kid's enjoying level C grid perplexors, get level D perplexors. Don't overlook Sudokus as great logic exercises.
Balance math and Barnes and Nobles Flash Kids, math for the gifted student books are also fun.
Kahn academy has a nice free online part which can help you see what they can and can't do, and encourage them to do some drill. The questions are quite repetitive, but they earn badges for progress, which really motivates my child.
We also have lots of games/toys, like a home made geoboard, a balance scale and pattern blocks.
