The "Myth" series are great romps, but light on world building, complex characters, and politics.

Particularly thinking towards Nerdnproud's "What is this thing called fiction?" side of the thread... Richly complex worlds with many characters and delving into politics and character depth can be as much sandboxes as they are their own content and language utilitization. It is a fertile playground to explore issues of philosophy and politics and human interaction outside of the expected/known world. Some people work best with many viewpoints and situations to develop robust ideas and fiction can help fit that need (because for some reason performing secret social experiments on friends and classmates is frowned upon .)