I've been following Igorsk's Reversing Everything Blog and today's
blog entry has a list of undocumented Kindle commands.
I haven't investigated everything but...
Alt-M on the home page brings up Minesweeper. Commands are listed at the top.
There's some kind of location finder when you're in the browser but I haven't been able to make it work.
There is also a picture viewer, read more on Igor's blog.