Ferrets are less destructive than cats, but they love to get into everything, so if you keep them loose you'll need to make sure they can't hurt themselves or your possessions. They love to steal small (and not so small!) objects and stash them under chairs and behind furniture. They like to chew on spongy, springy things, which must be kept out of reach or they'll swallow bits. Accessible boxes, bags, and trash cans will be crawled in, and houseplants within reach are liable to lose all their dirt to joyful digging. Finally, many ferrets tend to scratch and dig at the carpet. Naturally, these traits vary from one ferret to another, but they're all pretty common. If you're not willing to take the necessary time to protect your property and your pet, a ferret may not be for you.
Many people keep their ferrets in a cage or a ferret-proofed room whenever they can't be supervised. This drastically reduces the risks of digestive tract blockages from swallowing indigestible objects, injury, and escape. However, even if you plan to let your ferrets have the run of the house at all times, you'll want a cage at first for litter training and other kinds of training as well as for temporary use.
A metal mesh cage is probably the best choice. Many pet stores keep ferrets in aquarium-like enclosures, but they are not recommended as cages. They don't provide enough ventilation at the bottom, and your ferret will feel isolated from what is going on in the room. Most aquariums also aren't nearly big enough. Plain wood cages aren't recommended because the wood soaks up urine and other liquids, so getting the smell out and getting the cage really clean are nearly impossible. If you use wood, cover the floors with linoleum squares or coat the whole thing with polyurethane.
If you plan to keep your ferret caged whenever you're not home, and you'll be gone most of the day, a generous cage size is about 2 x 3 feet and 2 feet high (60x100 x 60 cm). A second or third ferret could share that size cage. Of course, a nice, big "condo" is even better, especially with lots of levels and hammocks to prevent falls from the top shelf. If you'll only be using the cage temporarily, such as when you're vacuuming or taking your pet on a vacation, 1 x 2 x 1 feet (30 x 60 x 30 cm) is sufficient for one or two ferrets, perhaps three. For trips around town, a shoulder or duffel bag equipped with a litter pan and mesh window works well.
One option is to make the cage yourself. It may be cheaper than a store-bought cage, and you can get exactly the size and configuration you want. Photos and descriptions of various types of homemade cages, as well as instructions for building one of them, are available. Of course, pet stores and catalogs have lots of cages, too. Multi-level "cat condos" are probably the most popular store-bought cages. Some people like the easily cleaned medium or large size plastic dog kennels, modified to make multiple levels, although others think that they don't provide enough ventilation or contact with the outside world.
In the cage, you'll want some sort of "bedroom" for your pet. A ferret won't be very happy sleeping on the open floor of a cage, even on (or, more likely, under) a towel or shirt, but any small cardboard box or basket works well as a bedroom. Old T-shirts and sweatshirts make excellent bedding, as long as they aren't too easily chewed to bits. Old towels usually work well too, though some ferrets tend to get their nails caught in the loops. Don't use wood shavings. The bottom of the cage can be covered with linoleum squares, carpet samples, or cloth cage pads.
Other than food, water, a litter pan, bedding, and a bedroom, what you put in your ferret's cage is largely up to you. Enough room to stretch and move around is important, and different levels, ramps, tunnels made from dryer hose or black drainage pipe, and so on will probably be appreciated. Hammocks made from old jeans or shirts and a set of metal eyelets are very popular for both napping and playing. Most ferrets get bored easily when caged and sleep much of the time, so they probably won't get a whole lot of use out of toys; they'd really rather be out playing. Just be sure nothing you put in your ferret's cage could hurt him, whether by catching a toe, being swallowed, or some other way.
Also be sure your cage door fastens securely, perhaps even with a small lock, because ferrets can be very determined and rather intelligent escape artists. Twist ties, cable ties, or bits of wire often work well for fastening down litter pans or some bowls; and clothespins and small bungee cords can be enormously handy for holding all kinds of things down, up, or closed.