Guinea pig cages need to be large enough to provide them with enough room to exercise and "play," ideally, a minimum of 7.5 square feet (30 inches by 36 inches) for one pig to 13 square feet (30 inches by 76 inches) for four4 pigs. Less than this, and it could lead to overcrowding and stress-related issues or obesity from lack of activity. Cages must also be escape-proof and not made of wood, since guinea pigs love to chew wood and could easily make an escape route for themselves. Plastic, wire, stainless steel or combinations of these materials are best. Wood is difficult to clean sufficiently. Glass cages do not provide adequate ventilation.
The flooring should be solid and easy to clean. Avoid wire mesh for flooring as standing on it for long periods can injure your guinea pig's feet. If using wire flooring, provide grass mats or other flat surfaces on which the guinea pig can rest. Whatever the construction material, your guinea pig's cage must have a latching door or lid, both to keep the guinea pig in, and to keep other pets out. The cage must provide adequate ventilation, and be tall and spacious enough to provide some ramps and activity areas. Finding a large enough enclosure is sometimes a challenge. However, if you let your creative juices run, and keep the above guidelines in mind, there are many choices. A good choice is a children's wading pool with a wire "wrap" around it. Cage walls should be at least 10 inches in height. Guinea pigs are subject to hyperthermia, so the cage should be located out of direct sunlight, and be kept at a temperature of 65-79ºF.
Many people take their guinea pigs outside for exercise in the warm weather. If your yard is safe, secure, and free from pesticides and chemical treatments (or you can section off an area that is), go ahead and give this a try. Monitoring is still important, as is ensuring the availability of access to shade.
A "hide" is especially important for your guinea pigs. Each pig should have their own. These can be made of heavy plastic, PVC pipe, or stainless steel - all easy to clean yet light-proof. Since this is a place to hide, glass is not a good choice. Food dishes should be heavy ceramic and not large enough for your pig to climb into. Guinea pigs are not very discriminating in their toilet habits and if the bowl is large enough, they may defecate or urinate in the bowl. The hanging type of water bottle with a spout is a good choice for a water supply. Guinea pigs tend to drink lots of water, so monitor their supply closely. A small "hopper" for hay is a handy way to keep it clean and up off the floor. For bedding avoid cedar and pine. The recycled paper beddings are an excellent and environmentally friendly choice. PVC pipe, ramps, and other explorable but sturdy pieces of "furniture" make good items to enrich your guinea pigs' activity area.