1) A large water dish is a must for both the ferret's health and as a great toy! Many ferrets enjoy splashing in water and some will even spend hours fishing toys out! A water source which cannot easily be spilled and is big enough for the ferrets to play in is a great thing. If you are concerned about protecting the flooring underneath, you can consider placing such a toy on a plastic boot tray - the tray will contain the water and protect the floor. Safe toys for putting in the water include dura rings, disks and balls which are made of hard rubber and float, kongs, nylabones, hard plastic baby toys (with no small removable pieces). Be sure to inspect any toys left in the room daily and discard them when they begin to wear. Rubber duckies and other soft latex toys are not recommended for unsupervised play.
2) Tunnels!!! Plastic drier tubes with the wire ends taped off (so long as your ferret does not like to chew duct or electrical tape) can be wrapped around objects such as chairs or cat trees, tangled into a mess or taken out along the edges of the room. Most ferrets LOVE 
to play in tunnels, and best of all, these tunnels are cheap and can easily be replaced when damaged! Empty carpet and hard plastic water pipes also work. As do plastic ferret tunnels.3) Crinkly objects such as cat crinkle bags, crinkle tunnels or even crinkle pants can often be found at most pet stores, and sometimes even at your local thrift store. They are much safer than plastic bags for unsupervised play, and just as fun for your ferrets!
4) Beds. Lots of beds. They can be as simple as boxes with blankets or as fancy as an octoplay - ferrets LOVE having a variety of beds to choose from.
5) Cat trees with low perches and soft beds can provide your ferret with quite a bit of entertainment. Most ferrets will enjoy rubbing against and scratching at the material of the cat tree, as well as curling up in the cubby holes.
6) Dressers. I know of one person that has taken old dressers and transformed them into fun ferret play zones. Dresser drawers are filled with bedding, crinkle paper and novel objects. The contents of the drawers are often rearranged and the ferrets LOVE climbing the dresser from within to see what novel objects are located in each drawer! If you are concerned about your ferret falling from a height, simply remove the top drawers.
7) Dig Boxes are always great, and the nice thing about having them in the ferret room is that if the ferret makes a mess with them, its less of a problem! I have two different types in my ferret room - one rice and one dirt. I alternate between boxes to keep things fresh - when they get tired of the rice, out comes the dirt and vice versa!
8) Litter boxes. In every corner.
9) Food. Food should be available at all times.
10) Their cage for when they are done playing.
