Adventure Time's creators have released a free browser game that lets players make their own levels, quests and power-ups. Mathematical!
Adventure Time Game Creator can be found here, and while you can only play as Finn or Jake at the moment, but new characters will be added over time.
Creations can be shared with others and while the app is ultimately aimed at kids, the show has a big adult following too so we won't judge you if you spend the rest of today playing around with it. Honest.