From the popular letsplaymath.wordpress.com blog, I must borrow a great game idea:
“Have you and your children been struggling to learn the math facts? The game of Math Card War is worth more than a thousand math drill worksheets, letting you build your children’s calculating speed in a no-stress, no-test way.”
Here’s the complete post (I particularly like the multiplication versions suggested): Factor War. First through fifth grade math teachers are advised to read it!