Explore Wolfram Demonstration Which Country is Bigger?, with which you can compare the land area of any two countries in the world. One of the images in the Slider Puzzle is a map - you can try to put the world back together.
There are lots of interesting projects listed on the "For Kids" index page:
To interact with the projects, you'll need to download and install the free CDF player on your computer.
Thanks, Graham, for reminding me about this great site!