For a gerbil or hamster?
Best housing for gerbils are aquariums (or sterilite bins) because cages with bars are to drafty (and make messes as gerbils kick out litter) and as gerbils chew on them it rubs the fur off their nose making it sore. Plastic cages are dangerous because if a gerbil chews and ingests the plastic, it can cause fatal blockage. Tanks and bins provide plenty of room for a gerbil to burrow in deep bedding, and are easier to clean.
Chew toys- Tree branches (maple, fruit tree, birch, beech, haze, willow NO PINE after you freeze them for 72 hours), paper towel/toilet paper rolls, cardboard boxes (ie: kleenex boxes, ramen boxes, shoe boxes, food boxes, popsicle boxes- if no chemicals and paints, removing glue), wood (nothing treated or finished), manilla folders, store sold wooden logs/treasure chests, cuttlefish bone, paper plates, computer paper
Toys- Jars (empty coffee jars, pasta sauce jars- gerbils favorites), wheel, rolling ball, large marbles, tubes
Cage decorations- Rocks, stones (after they've been cleaned), glazed figurines (nothing plastic)
Food dish- optional. keeps food together from getting lost, or gerbils can scavange for food in their litter
Houses- clay/terecotta plant pots, cardboard, store bought wooden house, hollowed out coconut shells
Levels- gerbils love levels- ramps, mazes, tunnels, bridges, stairs are all great out of cardboard, or bird toys,
Bathing- Chinchilla bath sand in a fish bowl, or small cardboard container. Place in cage for 5-10 minutes and then remove before gerbils decide to use it as a bathroom.
You can also dump lots of timothy or alfalfa hay in their tank, they'll love it especially if you mix treats (pumpkin seeds, saaflower seeds, oatmeal flakes, almonds, peanuts, walnuts, fruits, vegetables, sunflower seeds, dog buiscuits, etc) for them to forage for.
Food, you can buy a commercial brand and add fibers, dried fruits and vegetables, finch seed mixes, and more to it if you want, you can also give htem spray millet but it makes them VERY hyper. Best figures for gerbils are about 12% protein, 7% fat. Other sites say gerbils (as omnivores) need 22% protein, treats such as crickets and mealworms will always be appreciated.