I just stumbled upon this great website: www.mathisfun.com
This site has explanations, quizzes, games, puzzles and more to help with all kinds of math - from basic times tables and addition facts to algebra and polynomials. There is also Geometry help and explanations.
This would be a great resource to help kids retain their math skills over the summer, or as a review of different skills. It can also be a great supplement to your curriculum.