What are some of the best educational games to look at when designing one? I'm currently interested in designing with the goal of educating, but there aren't that many games that try to be educational, and I've only ever played a few. Zoombinis comes to mind, but that's so old I don't even really remember much about it.

To be clear, I'm casting a fairly broad net here, Age of Empires and other historical games usually have some good educational details. I'm looking for examples of great design where the educating doesn't feel like hard work, so many that are aimed at children would be excluded for this reason.

Ideally, educational games aimed at adults which make the learning fun and easy without focusing on it to the exclusion of game-play, but I'll take what I can get.