A while ago Terence Tao posted a a problem from his son's Math Circle. It is in his own words "surprisingly difficult":
> Three farmers were selling chickens at the local market. One farmer had 10 chickens to sell, another had 16 chickens to sell, and the last had 26 chickens to sell. In order not to compete with each other, they agreed to all sell their chickens at the same price. But by lunchtime, they decided that sales were not going so well, and they all decided to lower their prices to the same lower price point. By the end of the day, they had sold all their chickens. It turned out that they all collected the same amount of money, $35, from the day's chicken sales. What was the price of the chickens before lunchtime and after lunchtime?
Terry Tao seems to be forgetting some of his basic math as he spends most of time on super advanced stuff. He also has a post where he discovered the surprising fact that if you stay in your arrived airplane's seat until most people have exited, after the crowd clears you can walk off the plane more quickly.
By 8-9, something like the Japanese city-building card game Machi Koro is great for learning about variance, expected value, and optimization.