The water tribe island has a fully fleshed out plotline revolving around shady ascending and korra working out her feelings for tenzin, it goes on until you make it back to the air bender island, you can have anal with korra's mom, the girl half of the water tribe twins, korra herself, a blowjob by the spirit of his air bender past life and sex with the box spirit if you did everything leading up to this point right, there's also cameo's by the current elders' katara/the clerk/alleyway girl, and the snake spirit you can ask the snake spirit if it's possible for you to make snake babies with her and the answer is apparently yes. Oh, and also some info on what happens to the people the avatar possess while they're possessed.
Note: aside from that one game on the boat you don't need to play the cardgame at all it's completely optional.