Originally Posted by
Samwich
Ultra microscale worldbuilding here, but it's one of my favorite details of my campaign setting.
The village the players start in has a strong gnomish influence, because honestly, gnomes are best race. One of the features the original gnomes built is a fountain set atop a geyser, so that every day at a specific time, the geyser erupts and water shoots out of various orifices in the fountain. To make it a bit more interesting, on one end of the fountain is a small box nobody has managed to open for decades. If the players can solve a few puzzles, they can open the box and access the control panel for the fountain. They can use the control panel to inject all sorts of chemicals into the fountain water, altering it's properties to do all sorts of wacky stuff.
Like I said, it's the tiniest of minor details, but I had a lot of fun making it up.