Cartoon Network has released a free-to-play browser game where you can create your very own Adventure Time game! Adventure Time Game Creator let's you design levels, set up enemies, power-ups, and more. Right now you can only play as Finn or Jake, but more characters from the show will be added over time.
Whatever levels you make can then be shared with the others. I tried out some of the levels that were available right at the front of the website and they were somewhat enjoyable. Mind you what you're creating is very simple stuff, and I think younger audiences will have way more fun with this one.