You don't have to buy a fancy den for them. You can buy this at a hardware or home improvement store. These can be scrubbed clean with any kind of brush, any soap, and rinse thoroughly. Make sure it's completely dry before you put it back in the cage. These are completely unbreakable, and a piggy gnawing at the edges will not do it any harm.
Some people use terra-cotta flower pots as dens.
These need to be cleaned more carefully than PVC, and they are breakable, but they might be more attractive in the cage. Hedgies especially seem to like these. The bottom of these needs to be cleaned quite thoroughly and often. Many rabbits don't care whether they have a den or not, and some prefer to just use their cozy litter box as a place to sit and sleep. Toys Bird toys , made to hang on the sides of cages, can make excellent chew toys for guinea pigs.