I often find that when I am stuck for an idea, the easiest way to both buy time and still game is to choose a single monster(or group of monsters) that can really challenge the PCs and set up a classic monster-hunt/PC hunt, similar to such classic movies as Alien, Lake Placid, Night of the Living, Dead, etc. The idea being that the PCs should be fairly isolated somehow, relying on only their abilities for survival, unclear of when or how the monster will attack and looking for a way to kill the monster.