Steam is being weird, it doesn't recognise when I own a game either on the website or through Steam Client when it comes to store page or in the cart.
Also I would recommend not being fooled by Adventure Time, it's not a good game and just utterly empty of any real substance. At least from a game play perspective and someone who likes dungeon crawlers, I can't speak at all in regards to the show as I pay no attention to that.
Giana Sisters: Twisted Dreams on the other hand is a strong platformer that makes good use of it's "shifting" between two styles to keep you busy. While not the most challenging in terms of platforming skill in the style of Super Mario speed runs or Super Meat Boy, it will satisfy those looking for a PS1/Saturn/Windows 95/98 era type platformer with a twist.