The Sandbox campaign

Our group mostly plays published Adventure Paths. We're all busy professionals and don't have tons of time anymore laying out expansive campaign worlds with dozens of adventure hooks and encounters prepped. I don't have the spare brain capacity to memorize the stats of a dozen monsters (it doesn't help that 3rd edition+ monster stats are so much more complex then earlier editions). I would love to open up the world to everyone and say "Go play with what you want" but I honestly don't think I can handle that anymore.

Since I am trying to avoid a narrow path where the story drags the PCs from one encounter to the next I've decided to go with a mystery type adventure. The clues are hidden in various locations and its up to the PC to solve the mystery. Of course, events will going on in the back ground that will also effect the main plot. I don't want to script things to much since interesting things often occur during play sessions and it would be nice to be able to run with them.

I'm a bit concerned about pacing. I've been watching our CoT sessions and we seem to get through about 3 major encounters a night and so I've tried to plan accordingly. I've been trying to come up with some interesting encounter designs. Too often it's just fighting in a room. I can foresee some environmental obstacles coming into play which can either be a boon or bane.