Personally I don't allow any of the rats to run around outside, even while leashed. There's too much of a risk of them slipping out of a leash and running off. I also fear any illnesses and diseases they could pick up off the ground, such as when a cat happens to crap in that place 2 weeks before or if a rat runs through, etc.

Better to be safe, than sorry, I say! Instead, you can plant grass seed in your house and grow your own grass if you want them to have that kind of experience.
As for inside.. the rats run around on us, beds, counters, tables or in the rat room. There is also a little hallway outside the rat room that they can run in as well. I also have cats and make sure the rats freerange time is as ratproofed and safe as humanly possible. So far so good!

-Melina