I'd heard that National Geographic has a new almanac out and that they are having someone come to schools and use the almanac for a game where the kids do a Jeopardy-type set up and answer questions. That made me think that this would be easy to do at home as well.
It's so hard to convince kids to want to learn while they're on summer vacation, but if you make it a game and turn it into a type of competition for fun, everyone wins!
Here is the video about the almanac that shows how you can use it as a game. It's illustrated, but you totally get the idea of it.