If he is litter trained, some choose to use a replaceable carpet square (without little thingies he can chew and swallow) You can use newspaper strips as long as he does not eat it in large amounts (inc can be harmful) or wood shavings. Pine and especially ceder are not recommended. However, I use pine and have not had problems with it, but it’s up to you.  However, all agree aspen is safe, so if you wish to avoid the issue use that. Hay or straw are also an option, but again not everyone agrees whether these could be harmful. (Rabbit may eat soiled parts) Do not use sawdust because it is too fine and could get into his system.

My rabbit lives in a large dog crate and the black tray is shallow – so I use a folded blanket and wash it when he misses the litter box.

Do you mean flooring or cage bedding? What Harveybunny meant is probably bedding.

For flooring, I use puzzle foam mats. The surface isn’t exactly the spongy kind so it doesn’t absorb liquid if Kurotta accidentally pees outside of his cage or litter box. I also know people who put down area rugs or a fleece blanket for traction. Some bunnies tend to chew these though, so do keep a lookout for that.