One afternoon module for a camping trip?

Next week, I'm going camping with my wife, my brother and his wife, and my 9-year-old niece.. My brother asked if I'd prep a DnD game to play. My niece knows the basics and I'm modifying rules to make it easier for her, and I'm making a bunch of characters for them to choose from based on Gravity Falls characters.

I'm prepping A Wild Sheep Chase for playing out in the fresh air, but we have a chance of rain I'd like suggestions for another short adventure i can prep. The sillier the better!