Sam marrying Rosie Cotton
I thought that was one of the sweetest scenes in this whole movie :D. It was especially delightful to see the 4 guy Hobbits in the tavern again (sort of a callback to a scene in "Fellowship"), and now, after his big adventure, Samwise finds proposing to Rosie is nowhere near as dangerous or risky compared to what he's been through, lol. My brother tells me that Rosie said "Yes" because she'd had her eye on Sam for a while, he was just too shy to speak to her. From what I've heard, they had a very good marriage, and had 13 children. (Hobbits are very prolific and for some reason, have no issues having lots of kids).
Sam even kicked those Sackville Bagginses out of Bilbo's old house and forced them to pay for the damages (they were squatting there while Frodo was away saving the world). He even became mayor of Hobbiton. Talk about a happy ending for Sam :D