Some episodes have different versions of the opening theme. In the Fiona & Cake episodes, it is a gender swapped version of the regular intro with a woman singing the theme song. The episode A Glitch Is a Glitch, the opening is CG animated. In Food Chain, the opening has all the characters with Finn and Jake's faces and the theme song is sung by a different singer as well.
In every episode, during the opening credits, when we see Jake's portrait with "Jake the Dog" written next to it, Jake winks. This is something a human would do. Similarly, a couple of seconds later, Finn barks and sticks his tongue out in a dog-like fashion, despite "Finn the Human" being displayed next to the picture of his head.
Various animals/insects such as a bee and a worm fly around or move on the grass in the background during much of the ending credits in most episodes.