Anything vinyl, like the tablecloth, shower curtain, etc. can be good. You have to make sure that your bun doesn't lift it and chew it though.
In my cage, I am using vinyl flooring, so it is much much thicker and more resistant. We got ours for 13⬠for a piece of 4m2 so that was a good buy. It comes in different textures, colors, and prices. It's meant to replace carpet and is used in kids' rooms a lot. It's not like vinyl tiles that you stick, although that is an option as well. If you put anything vinyl, I strongly recommend it being a bit bigger than the area so your bun cannot chew the edges. You have the added advantage of running it up the sides a bit as well to prevent pee from seeping from the corners
You can also use ceramic tiles, glued to MDF or plywood (we glued ours with construction glue) and seal the edges with silicone.
I also used a heavy-duty tarp (that strong blue plastic tarp found in DIYs) and it worked well but I was worried that he might chew it so that's when I got the vinyl. If your bun doesn't seem interested in it, it can be an option.
Some people use carpet. I don't use it myself because I prefer something easy to wipe down that isn't absorbent. On the other hand, if it doesn't absorb and your bun makes a bit of a mess, you are sure that your bun will run through it and have wet paws. Lol. With my allergies though, anything soaked with bunny urine is a big no no so it's just not an option for me.
I have seen people use those foam puzzle pieces. I used the big foam pieces as well at one point but my bun was way too interested in shredding it and chewing it. I wasn't sure if it was safe soI just felt it had to go. Plus, if your bun isn't litter-trained, it's really hard to clean that because the urine will seep through. You definitely would have to put something under it as well because pee can seep through the joints and run onto your floor/carpet.
Cardboard/newspaper is always an option. I would always use something waterproof (vinyl shower curtain, tablecloth, tarp, garbage bag,etc) under anyway though becauseit can get soaked through easilyIt will require frequent replacing but it's cheap and bunnies can do what they want to it.
Those are the ones coming to mind right now...