I keep a lot of old RPG books around that aren’t in PDF.

GM Tip: Never throw an old RPG book away. The same can be said for old modules, even homebrew ones. Please believe me when I say, “Never underestimate the value of an old module.”
The old adage that one man’s trash is another man’s treasure applies in this case. Take an old Marvel Superheroes Module like, Reap the Whirlwind, take the word “mutant” and change it to however supers are referred to in one’s own campaign and convert/add stats accordingly. Viola! Instant adventure for a couple of nights.
It’s one reason I favor simpler superhero games such as ICONS. They’re much easier to convert old Marvel and DC modules with. Games such as Mutants & Masterminds or Champions are a lot stat intensive and porting in all the baddies takes time and temperance.
Excavate those buried gems!
This trick works for more than just the superhero genre. Fantasy games, such as D&D, have far more modules already produced. Sometimes it’s as simple as updating the monsters and loot to the most current edition, which could involve some number crunching. However, if one is to choose a rules lite system such as ICRPG or Easy D6, conversions go very fast.
The same can be said for converting non-D&D adventures into other game systems. I know someone who really likes the Warhammer Fantasy setting, but doesn’t care much for the rule system. Solution: convert everything to D&D by approximation. One of my current projects is pulling old Basic D&D modules over into Dungeon Crawl Classics for my own use.
I’ve seen the Star Frontiers Crash on Volturnus module used as a Star Wars D6 adventure. I’ve seen Call of Cthulhu modules run in Beyond the Supernatural and D20 Modern. (Call of Cthulhu actually has a D20 variant, but the modules were original system.)

Wait, there’s more!
If you really want to expand your horizons as a GM, you might consider running modules across different genres. Call of Cthulhu investigators stumble into what looks an awful lot like a D&D dungeon full of monsters, riddles, and deadly traps. Star Wars characters have to go up against a rogue group of stormtroopers that have broken off of the Empire under the leadership of a maniacal megalomaniac with a skull shaped red mask. D&D characters suddenly find themselves up against Cthulhu cultists.
The possibilities are truly endless. If you’re a low prep GM and you have become adept at “winging it,” then this style of grab-n-go module prep might work very well for you. I’m more of a high prep GM, but I keep a LOT of old adventures around, especially D&D, that I can pull into my current game to run as a side trek or maybe as a one shot if key players are missing.
Thanks for stopping by. I hope you found this useful. Happy gaming!