I love the idea of free range as well. My piggies are not litter trained so there's no way it would really work with them, and my place always has trouble somewhere.
My tip would be to let them roam free during supervised floor time for a few hours. Once they get bored, watch what kinds of trouble they get into to determine if you forgot anything when you piggy proofed the area. What I noticed by doing this was that when I blocked off an area, it was almost like asking them to try to find a way in -- suddenly that was the most interesting part of the room. Also, ANYTHING on the floor will be chewed and swallowed. My friend walked by eating and a piece of food fell, and my piggy raced me to it and had it swallowed by the time I got there. Luckily, the cooked red pepper didn't do her any harm. My other piggy got to a cord. He chewed through the ground wire. Luckily, he never made it to the important wires. And I watch my pigs like a HAWK.
Don't keep things leaned up against the walls. My piggies think they're good hiding places until they push to give themselves a little more room, and the whole thing topples over.
Those are just incidents that I've had that have helped me better piggy-proof my house for floor time. I'm sure there's still a ton of stuff that my piggies could do if they had a whole day and the whole house/room to themselves.
Also, mine chew on the wooden furniture. I shoo them away when they start chewing, but keep in mind that treated wood isn't good for them to be chewing, so if you have any, expect it to be devoured.
Really just expect the worst and plan accordingly. The piggy free range room, as partygirlha said, shouldn't have anything within piggy reach that you wouldn't put in their cage.