I've volunteered to lead a programming club at my son's middle-school. We've covered a lot of fun topics, including showing them how to use a game engine to build 3D interactive environments. One thing they said they would like to see is how to use scripting languages to enhance and extend games.
I know I have heard that a lot of games use Lua as an embedded scripting language, and we have talked about Lua previously. I've found that the kids like to see something fun and flashy and are less interested in practical things. Of course I try to sneak in some learning too.
What game can be recommended that would be fun to extend via scripting?
It doesn't have to be Lua scriptable, but that would be nice. Bonus points if the game is free or open source so the kids can download it at home. I prefer Windows, but Mac is fine too. Game needs to be teen friendly.