I love Fionna and absolutely adored the Sailor Moon reference in that episode (when she goes to the dance wearing the Princess Serenity dress) and I squeeled when I first saw it.
I also find the Ice King to be a very interesting character, and enjoy the back stories that are coming out in newer episodes that go into his past during the Great Mushroom War and how he came to be.
I love looking past the adorable characters and cute storylines into the actual history of Ooo.
It's pretty deep once you start researching; nuclear war, implied genocide.. Scary stuff, but it's for kids whom enjoy the action instead of digging into Pendleton's mind hah :3