What is the best way to thank your GM?
My personal take on this: find out what they want to play, and then run it for them. Most GMs like GMing or they wouldn't do it, but there are those of us who would appreciate the chance to play now and again. Plus, it's a great way to try it out, if you've never GM'd before. You have an audience that is invested in your success and a clear goal to press towards, as well as a ready resource if you have questions about how to do something or what to do next.