Mice need a box or house for a nest and preferably a wheel to run in. You can use your imagination with the nest box, as long as you remember that plastic parts are not safe, the mouse toys should not have sharp edges and of course there should not be anything poisonous in them (paint, for example). There are several different alternatives available in pet shops; wooden, ceramic and plastic. The problem with wooden nest is that the mouse urine will soak in it and therefore you should wash a wooden nest often. Ceramic nests are expensive, but they do last a lifetime - and not just your mouse's lifetime! Plastic ones can collect humidity inside the nest and mice tend to chew holes in them. Splinters of plastic are not healthy for mice if they happen to swallow them. You can construct a mouse nest yourself as well. Earthenware flower pots make great nests and the mice love them. Make a little hole in the side of a flower pot and turn the pot upside down. Be careful when making the hole, as the pots go easily into little pieces. Empty shell of a coconut is a good and fun nest too, especially if you hang it from the ceiling of the cage / tank.
The wheel has to be a safety wheel. The wheel shouldn't have bars, spaces or holes on the running area. The wheel should stand on a "leg" attached to one side only so that the mouse's tail won't get squeezed. There are plastic as well as metal wheels available and both do fine. Metal ones tend to get rusty after a while and they tend to make more noise. There are also wheels that are attached straight to the wall of a cage. The wheel must be large enough, surprisingly large. This is important, as the mouse's tail can get permanently curved over its back if the wheel is too small. Proper size for a wheel is one where the mouse can run without its back curving too much. With a group of mice you need a large wheel, as mice love to run together with other mice.