This is what I call "evergreen" material: keep a bunch of somewhat generic battle maps around as potential drop-ins. When the players decide not to go to the dungeon you had in mind for that week, or clear it out and there are still hours of time left, or whatever then you can pop one out, quickly populate it (undead denizens? Stormed by goblins? Trolls? Whatever's appropriate for the area and difficulty), and not be flailing for something to do.

I'm a bit unsure of how to post this, though. It's made in maptools; the overview is:


but there's a lot more detail if you zoom in:


I figure the overview shot gives a good feel for the architecture.