I'm going to try my hand at writing an old-style adventure game, and I think I've got a good mystery! It'll be a riff on The Eight Strokes of the Clock by Maurice Leblanc, a 1922 book. It has the bonus of being a French book, so not many English readers are familiar with it.
The mystery is that two people break into an old, abandoned house. No one has been inside the house for twenty years. As soon as they enter, a grandfather clock strikes eight. And that's a mystery! How could the clock still be going, after all this time?
The solution is that there was a telescope hidden in the clock, which blocked the pendulum from moving. When our heroes smashed the door open, they unknowingly dislodged the telescope. I hope this will translate into a logical chain of adventure game puzzles. That is, you've got the puzzle of entering the house. The clock sounds, leading the player to the "search the clock for a telescope" puzzle. Getting the telescope leads to the puzzle of figuring out where the telescope can be used.
I have no idea how to format adventure game scripts, though. I looked online, and one person said that you have to use Excel. That seems overly complicated, but then again, I usually write everything in Notepad. My wife makes fun of me for not using Microsoft Word like a normal person, but I can't seem to use Word without my laptop fan going on full blast.