I'm running a game of Fate soon, but I love the Pathfinder setting and know I can trust Paizo with good quality. So what I've decided to do is buy a Campaign Setting book of a cool region and have my adventure take place there.
I want it to have lots and lots of room for wilderness exploration, hidden artefacts, secret dungeons and ancient mysteries to solve but with places for interesting NPCs with whom the players can interact. Not really glorious crusades and paladins, more a place for good-hearted scoundrels or wary rangers. Hope that makes sense.
I was thinking the River Kingdoms, but I'm not sure if there are many hidden secret locales. Any thoughts much appreciated! :D
Also sounds like Varisia. They don't call it "Birthplace of Legends" for nothing.
My immediate thought is Varisia, which is more or less the "Here be adventure!" region. While it's been settled, it's not fully explored, leaving a lot of dungeons and ruins from ancient Thassilon to still be plundered. It also has a very diverse group of locals (dwarves in Janderhoff, elves in the Mierani Forest, gnomes in the Sanos Forest), as well as different humans (Varisians, Taldans, Chelish, Ulfen, Shoanti being the most common) and halflings.
It's a very well fleshed out region, with books associated with each individual city state (Magnimar, Korvosa, Riddleport), as well as a single book detailing the region at large.
I really think Varisia would be your best bet. It's a fun region with several city-state-esque places (Magnimar, Korvosa, Riddleport, Kaer Maga), the ruins of an ancient empire (Thassilonian), large expanses of relatively untracked land, traveling caravans, a mafia (the Sczarni Varisians), and much more. The River Kingdoms could work, but I think Varisia has a better mix of the ridiculous and the lethal.
EDIT: What they said. :)
Ah, Varisia! I really haven't looked into the region much, for some reason, so that's soon to be rectified! Thanks so much for the responses.
I was wondering if the Mwangi Expanse would be cool, thoughts on that? Though it's really good to know that Varisia's had so much written about it, I think more than the Expanse.
Mwangi Expanse sounds like it could work also, but as far as I can tell it has less source material on it (although a large part of the Serpent's Skull Adventure Path is set there). If we're getting into regions that are have more limited representation like that, also consider the vicinity of Katapesh (Legacy of Fire is set there, but beware that you'll need to do conversion from D&D 3.5 to Pathfinder), the northern River Kingdoms/Stolen Lands (Kingmaker -- YOU get to make the kingdom that goes there), and Osirion (Mummy's Mask; also has Osirion, Land of Pharaohs for further source material).
Varisia fits your criteria, except for one point: there is more than one sourcebook! Magnimar, Kaer Maga, Korvosa (3.5), the Riddleport gazetteer in the Second Darkness AP (with a smaller version in the SD player's guide), plus various shorter entries in other Campaign Setting sourcebooks (Towns/Castles/Lost Cities/Lost Kingdoms of Golarion/the Inner Sea) and shorter gazetteers in various APs (Rise of the Runelords, Curse of the Crimson Throne, Second Darkness, Jade Regent, and Shattered Star). The player-friendly sourcebook is Varisia Birthplace of Legends.
If you're hankering for more southerly climes, Osirion might be an answer. Osirion Legacy of Pharaohs is the main PF sourcebook for it, plus some really-GM-only information in the old 3.5 player's guide (Osirion Land of Pharaohs) and again a scattering of shorter entries in Campaign Setting sourcebooks (Lost Cities of Golarion and Lost Kingdoms). The recent Mummy's Mask AP also has some relevant regional information in the gazetteers in the back of each issue. The player-friendly sourcebook is People of the Sands.
As always, the Inner Sea World Guide is very useful for the GM as an introduction to the whole of Golarion (mainly the continent of Avistan plus northern Garund), and the Inner Sea Primer can help players (mostly if they don't want to come from the region where you've set your campaign).