For session prep, I use The Eight Steps of the Lazy DM by Mike Shea at Sly Flourish. Admittedly, the Eight Steps have turned me into a bit of a Mike Shea fanboy, but that’s because the system has had a big impact with making my game better. It is a system that focuses on prepping the most important things that will help you run the game at the table.
I wanted to suggest the Eight Steps because I think it circles very closely to your mad libs idea and also follows the prep situations not plots concept. Where mad libs is a system where the players fill in the sentence blanks with their own noun or verb, if we follow the metaphor, I would equate the Eight Steps with prepping a list of nouns, verbs, subjects, and adjectives, which you then use to help the players make their own sentence.
For example, in the Eight Steps you would prep likely locations the players would visit, secrets and clues they might learn, and potential monsters they could encounter. That’s all pretty standard fare, but a key concept is that all those things are decoupled from each other. There is nothing saying that this monster will be encountered in this location, or this secret will be learned from this object in this location. Instead, you plug the components you prepped into the adventure as the players unfold it.
I could gush further on how much it’s helped me, but I won’t do that. Instead I’ll encourage you to check it out because it sounds like it might work with your brain.
Yup, that’s the book. There’s a lot more in there than just the Eight Steps but it’s worth it just for that in my opinion. Have fun!