To show one of my group's younger players what "old school" D&D was like, I dug up the old 1st edition Players Handbook, DMG, and Monster Manual and improvised a quick dungeon crawl. We were trying to give the young'un context with which to appreciate the advancement and improvements of the...
https://www.drivethrurpg.com/product/285021/Modern-Dungeon5
I like this one. It's not tied to any particular setting so you can build just about any kind of campaign with it.