Loads and loads of things! Lots of people lament growing up, but I think it's great to be able to think and do childish things, while having the autonomy of being an adult.
I love swimming pool waterslides, and will go down them again and again. I can't wait to try the new indoor water park in the Midlands, holding out until after Christmas!
@TheDuchessofDukeStreet I like Ladybird books too, and scout out the old ones in charity shops.
I like to wear childish shoes, such as elasticated plimsolls, Velcro trainers, or Mary Janes (the "Red or Dead" style). Also onesies (not in public though) - I'm a bit sad that they weren't a thing when I was a child. I haven't yet tried wearing dungarees though.
I get really excited about my presents; only three days to go! Before my birthday or Christmas, I wind my DH up with "pleeeeeease will you tell me what they are??!", like I did with my parents. He teases and indulges me by wrapping them in front of me, which I love, but there's something about it that's really unfair: I can't see what they are, because I'm blindfolded!!!! 
Actually this was my idea, and is a long-standing tradition of ours.
A bit less funny is that I'm a bad loser of games: I curse and mutter if my netball team is losing a game badly, and I avoid the high-stakes games such as Monopoly.