How to Multiple Campaign Settings Distinct from one another

[spectresghost]

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 distinctive flavor. You could use the Moonshae Isles, Kara-Tur, or Maztica if you want to use a different cultural idea than the normal high middle ages-style places of most of the world. If you want to keep to the iconic places that are more familiar, there's always the Dalelands/Zhentil Keep area, Thay, or Waterdeep and the North. If you really want something different, go for Myth Drannor or Anauroch before they fell into ruin!

For Krynn, I would use either the Sea of Istar and the minotaurs, one of the Towers of High Sorcery, the High Clerist's Tower, or the city or ruins of Nekara if you're putting things during or after the War of the Lance. If you want to go back in time, put them into the city of Istar when the Kingpriest is reigning, one of the elven kingdoms, or Thorbardin.

 

[Mark]

You could also pick out some of the iconic dungeons from the various settings and have the spheres each hidden in one of them: Tomb of Horrors, Temple of the Frog, Dragon Mountain, etc. It might take some system adjusting to bring them in line with your game but you could also do some other movie, television, and legendary representations, like having one of the spheres lodged in the pommel of Exclaibur in an Arthurian setting, or put one in the Ark of the Covenant from Raiders of the Lost Ark, or have one be the heart/power source of Pikachu and need to be cut from its writhing body. Just spitballing here.

 

[AdmundfortGeographer]

For Oerth, I'd suggest considering doing it during the height of the Greyhawk Wars. Iuz's demon-led orc and human armies are expanding over the central third of the continent. The Great Kingdom is coming apart and going to war against itself as the Overking turned the most important advisors into undead animuses, razing some regions, and calling upon the aid of devils. The armies of giants come out of the Crystalmist Mountains swallowing up two whole states, Sterich and Geoff. The goblinoids and orcs of the Pomarj finally band together and invade the the dwarf nation of Ulek, and march up the Wild Coast towards Greyhawk city. The Scarlet Brotherhood's assassins strike across the Flanaess crippling some states while the Brotherhood imports armies of savages to conquer the lands of the Sea Princes.

I mean, it seems like the big three are more like scenery for the greater plot, so the Greyhawk Wars is probably the most distilled example of all that Greyhawk has going on.