For Greyhawk, I would use either one of the iconic modules set there or the Free City itself. If you're ambitious, what about using either the Temple of Elemental Evil/Hommlet area or the ruins of Castle Greyhawk.
For the Forgotten Realms, there are several places that stand out and have a very...
I was torn between suggesting OD&D/Swords & Wizardry and Basic/Labyrinth Lord. I think an old-school mega-dungeon is easiest to play with old-school rules. I ended up choosing LL because that's what I'm going to use to run the Keep on the Borderlands and, eventually, my own mega-dungeon.
I like...
If you're looking at jumping back and forth, you might consider a system that can accommodate both time periods. For that, I would go with either the D6 system (Adventure and Fantasy for the 30s and medieval period respectively) or Savage Worlds. SW is very customizable and has a ton of good...