Untreated cotton is considered “safe” as long as the rabbit isn’t regularly consuming it. I know many people opt for fleece stuff as well because rabbits tend to chew but not eat it (my observation from my Fable, who is a big chewer as well). I don’t know if fleece is specifically OK to break down in their stomach or not, but I know cotton is usually well tolerated.
Timothy and sea grass mats can be both engaging and provide traction. Depending on how slippy your floors are, you might consider getting a large sheet of cardboard to put down first, and that can provide a good amount of traction so things on top don’t slip around. It can also be an added measure to protect your floor, though once it’s peed on, you can’t really clean it.
Low-ply/very tightly woven carpet squares could be good too. You can cut them to size, and my recommendation is always to cut them so they go beyond the pen wall a bit. Rabbits who are chewers (like my Fable) can get attracted to chewing and pulling the carpet squares since they have rubber bottoms and I assume it’s fun to unravel them, haha. So it’s important sides aren’t exposed and they all fit together nicely. In terms of the material, I haven’t been as observant because, since it’s so tight/low-ply, there’s no way for her to bite down and pull out carpet strands to eat (like my apartment carpet…)
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.