Try a sandbox. Ditch the stories, grand or not. Just create a setting, and ask the players what they want to do. Wing it depending on what they want. Don't try to nudge them a particular way, don't try to guide them into some complex plan, just let them go where they will and adjudicate how it goes from there.
A dungeon or even Adventure Path does sound more like what you're looking for. Low-level dungeons are fairly predictable and can really cut down on the amount of prep time you have to do.Actually I think that you help me less. Reason being, I think the larger possible environment stresses me out because there are some many external factors that I need to describe and then have the PCs interact with. Usually throwing me way off any actual plans I might have laid. I think a pure dungeon environment would allow me to slowly work on my ability to run a sand box style game, or at least a much more open one.
Very hard to say. A haunted castle aboveground can look feel and function, while in play, just the same as an underground dungeon. A pocket of air deep underwater can function like a cavern...but isn't. And so on.
I chose 51-60 based on ad hoc recollection. I am assuming that anything indoors, and hence having reasonably defined boundaries and apertures (walls, doors/doorways etc) counts as a dungeon. I think this is in part system-dependent: I am GMing 4e, and 4e likes battlemaps, and battlemaps like boundaries.


(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.