Rabbits
As with almost any exotic pet the cage should be good sized. The bigger the better! There should be room for the rabbit’s food dishes, litter box, toys, and plenty of room for exercise. Rabbits should have freedom to run in at least a small area of your house. Leave his cage open so the rabbit can come and go as he pleases.
A cage should be the rabbit’s special place. Your relationship with your rabbit will depend a great deal on the respect you show the rabbit and cage territory. A rabbit will poo in their cage with a few pellets to mark their territory. This is not a breakdown in litter box training. Clean his cage while he is out of the cage. Allow him or her to enter and exit with their free will. Gently guide him into the cage. Don’t grab him to take him out of his cage.
Iris Plastic Rabbit Cage
Cages with wire floors are not good for rabbits. Rabbits don’t have pads on their feet and the wire floor will injure their feet. If you do have a cage with a wire floor provide a board for him to sit on. Keep the wood board clean and dry. I like the commercial cages for rabbits (example above). They are much easier to keep clean and to sanitize. I believe on the same page if you click the link above you will also see the two floor rabbit cage. They seem to like these pretty well. There are two of the two floor cages, one is much too small. The second one will also work with, ferrets, guinea pigs and chinchillas. Heck, I will so go find a picture of it, because it is kind of neat.
Super Pet Welcome Home Rabbit Hutch, Medium, Blue
Rabbits can have free run of the house once they are litter box trained. The house must be rabbit proof. If the house is not completely safe, the rabbit should have constant supervision the entire time he is out of his cage. The rabbit should be introduced to free run of the house gradually. Once he is litter boxed trained more area can be added
Tarantulas
Many exotic pets can use a fish aquarium and they can make good enclosures. The top should be screen, cheese cloth or mesh so that the aquarium has ventilation. Tarantulas are excellent climbers. Make sure their cage is secure and escape proof. You do not want other's harassing your spider or any harm to come to your spider.
I like the readymade terrariums, I found one that has easy access, good ventilation but yet holds the humidity. 
Zoo Med NT-3 Naturalistic Terrarium 18in x 18in x 18in
Tarantulas need humidity. Damp vermiculite works or you can use potting soil or a mixture of sand, soil and sphagnum moss, preferably a product such as Eco Earth or Repti Bark. Make sure the items you use are clean and sterile so bacteria or mold doesn't cause a problem. The material should be supplied to the point of 1/3 of the way up in the enclosure. This material is necessary; the spider needs humidity and moisture and needs to build his burrow. Do not use any cedar shavings or pine in the enclosure; it is harmful, in fact poisonous to the tarantula.
Dressing the cage up is not necessary, except for your aesthetic purposes. However, be very careful what you use. Sharp rocks if fallen on can cause a serious injury. A tarantula can fall and get impaled, also no cactus.
Keep the tarantula’s enclosures clean. The caging material need not be cleaned out more than a few times a year if you keep up on the housekeeping religiously. Insects that you feed your spider can be a bit messy when the spider is done consuming it so clean it up. The spider's excrement can be cleaned up with a tissue or paper towel. Keeping the cage clean is important in that mites will feed on the remains, also mold and mildew can form.
Keep the cage out of direct sunlight. Glass containers especially can get very hot which can kill the spider. If the container is to warm it can also dry out the humidly that is necessary to the spider. Added lighting is not needed and can be harmful. However, a black light or a infrared light is fine and is very nice to be able to watch your spider at night.
So many animals and so many enclosures but there are many commonalities also. The common factors are the bigger the cage the better (in almost all cases), safety, ventilation, the proper gauge wire for the type and size of animal, non-toxic paint, easy to keep clean and sanitized, secure closures and no small parts to ingest. Pets are very special and cost should not be a factor in purchasing or building a cage. Curious fingers and nimble toes a common factor in all pets!
