Logic puzzles are another fun way of feeding a math-talented child's need for stimulation. Childhood favorites of mine include Raymond Smullyan's "What is the Name of this Book?", and others, Martin Gardner's many math and logic puzzle books, and the Dover Recreational Math series. I also like Greg Tang's math picture books. I believe there are other recommendations in the Recommended Resource forum.
While I agree that afterschooling can result in insurmountable distance between instructional level and available instruction in a conventional schooling situation, one should also consider the possible damage to math enthusiasm when one is constantly forced to do math far below one's level, without any outside relief. Some children become convinced that they are bad at math, because they are bad at maintaining interest or attention in unchallenging drills day after day (which can also generate errors that, ironically, keep them out of the top math group in a class).
