One of the best ways to come up with a campaign setting is to get down to ground level and imagine the miniature landscape in one's immediate vicinity scaled up to 100 or 1000 times its actual size. Let's call this method, 'Small Worldism'.
Earlier this week I visited Iwaisaki, an cape of unusual rock formations off the coast near my wife's hometown. In an earlier life, she and I spent hours swimming in its stunningly clear waters, occasionally stepping back onto dry land to lazily drink beer and sunbathe. Now, with two young kids in tow, we mostly spent our time there chasing them around making sure they didn't slip or hurt themselves. But I did manage to take some low level photos to illustrate what I mean.
First, then, the overview. What we see here is clearly a vast lagoon, perhaps dozens of miles across, and surrounded by mighty rocky cliffs. At the base of these cliffs are many sea caves within which can be found entire city-states - their inhabitants trade and war with each other with armadas of vessels that continually traverse its vast expanse. (There also, naturally, here and there lurk pirates, too.) Higher up these cliffs lurk monsters, dragons, harpies, and the like. In the depths of the lagoon are crab-men, sahuagin, aquatic elves, tritons - the whole marine shebang. And on top of the cliffs are huge expanses of arid, barren badlands populated by savage tribes, outcasts and outlaws, and dotted with - natch - lost civilisations and ruins.
And so the thoughts go on. I daresay you could have as much fun in your back garden, though undoubtedly wild or rural areas are going to provide the most inspirational ammunition.