Astral Sea

The astral plane is described as the transitive plane and is largely empty apart from a few floating rocks. But the rocks in this map give players or anyone using the map a place to ground themselves in and come up with interesting encounters. I can certainly imagine this to be the kind of place a crazed wizard might be hiding out or where a lich has set up a hidden laboratory. You could even use this map as a demi-plane for your high level wizard players! :)