What is a fun childish thing you still do as an adult?
For me, it's sticking my hand out of the window of a moving car. Whether I'm driving or not, I love having the windows down and waving my hand in the wind. I've loved it since I was a child and I'll never stop doing it!
During spring time when snow is melting I like to dig small ditches to drain puddles and watch the water flow. Some of my mountain bike trips take twice as long because I can't help but stop for every nice puddle
EDIT: Also run! From my house to car or shed to house etc. I could walk but running is faster and more fun. I don't get why adults only run to excersice.
I did this at a beach recently, which had a slow trickle of water from a nearby wetland. I dug it out and made the little creek run into the ocean. My SO sat on a log and was watching me, amused x3
There's an even bigger one down the beach, but all the kids use that one and I feel a bit too self conscious as an adult to play in that one (unless I have a lot of friends who also would like to try!)