My goal with the game I'm developing is to demonstrate what one person can produce with effort and patience. I want to do it in a transparent way, so students in the future can look back over my material and journals to understand the process I went through. In that light, I want to open source my game, so that people can see how I achieved effects or organized my code, or whatever. The idea is that people can view the code, and tinker with it, but the code isn't meant to really be built upon. I don't mind if someone releases a game based on some of my code, but I don't want it to be a commercial license. What license should I use?

Take a look at the MIT or zlib licences.

Both MIT und zlib license don't prohibit commercial use.

In practice, the GPL will make commercial use imposible, since no one developes commercial GPL-licensed games. But it will also require non-commercial developers to release their work under the GPL, what might drive away homebrew developers.