If you don't have any safe areas to let your rats out on the floor, the free range cage is a great way to allow them a bit more freedom while still keeping them safe. The picture to my left shows the free range cage I have set up for three of my girls. The cage came with a wire grating on the bottom, which I removed, but I saved it as it fit perfectly over the top of the cage. I cut a small hole it in, just large enough for the rats to fit through, and attached a bird jungle gym and an igloo to the top (I removed the igloo for the picture). As you can see, the rats can easily climb up to the top of the cage through the hole and play up there. They can also climb down the cage on the outside (I usually leave the top door open). When I can't be there to watch the rats, I remove the top grate and replace it with the original top for the cage. I'm home a lot so the rats get to spend a lot of time out of their cage running around.
A free range cage should only be used when you are able to watch your rats or are at least in the same room and would be aware if one of them fell or jumped onto something else. The free range cage only works if it is on a high enough base (at least two feet) and is not next to any other furniture to which the rats may jump to or chew on.
If you have a room with enough empty floor space, you can build a simple enclosure to allow the rats to run around in. These can be built with many different materials, from hardware cloth to wood to storebought enclosures. The enclosure should be at least 3 feet high, depending on how active your rats are and what it is made out of. If it is climbable, it will have to have a ceiling or they will climb right out. No matter what, you should always watch your rats while they are out just in case they manage to escape. You should also keep other animals away from the enclosure. If this enclosure proves safe enough, you could even take it outside and allow the rats to run in the grass.
There are some commercially designed rat leashes for another type of sequestered free ranging, but often rats do not like wearing them and will just sit there and not move, or will try frantically to escape it. They are worth a try if you would like to start using one, but if a rat wants to it can almost always escape from one, so it should only be used in a place where it would be safe for your rat to be free if it managed to get loose.
Free Ranging on the Floor
If you have a room you can rat-proof, free ranging on the floor allows the rat the ultimate freedom and space. To rat-proof a room, remove any electrical cords, furniture with small holes that may be crawled into, anything that the rat may crawl under and you wouldn't be able to get him out, and any potted plants. If your floor is carpeted, you may want to cover it with a sheet or something waterproof, as the rats will pee and poop while they are running about. A hard floor can simply be washed when the rats are done playing. This kind of free ranging really only works best in a room that is nearly empty, as rats can fit under or inside just about anything, and they often chew on furniture. It is also easy to think your room is rat-proofed when it isn't, and you may be disappointed when your rat escapes under the door or fits into the hole you thought was too small. No matter how rat-proof your room is, always watch your rats when they are out on the floor and if they are going to be out for more than an hour provide them with a water bottle or dish.