I made a fleece forest for Olaf about 6 months ago and since he loves it so much I thought I would share with those of you who may not be familiar. It is essentially an open-ended NIC cube with strips of fleece hanging down inside of it. They are popular with guinea pigs as they love to hide in the fabric. I noticed Olaf loved to run through the curtains and thought he might enjoy this. Now he spends lots of time nibbling, nudging, and lounging in his little forest.

The link below shows you how to cut your strips without measuring. I am notoriously bad with a ruler so I like this method but there are plenty of other tutorials out there with clear instructions and measurements for those who prefer them. I used (3) NIC grids to make a frame, cut fleece with strips to fit down into the grids, and used a leftover piece to drape over the top and sides of the cube. You could also just tie strips to the top of your cage for an easy forest!
Directions to make your own: https://www.guineapigcages.com/forum/threads/76054-Easy-Pesy-Fleece-Forest!
Note: If your rabbit tends to ingest fabric, please be cautious. Olaf nibbles on the strips (almost like grooming) but never removes any material. Fleece is good for this project because it doesn’t have threads nor does it tear easily. That said, you know your rabbit best. Be safe!