At first I used white paper towels. Forest being a joey didn't bother it. I could keep a close on his poo and pee.
When I got Jenny, she being an adult, would shred the paper towels. I was afraid she'd eat it, so I removed it. Tray only. I wipe it down with baby-wipes every other day.
Forest has now decided to "play" with the paper towels, so I use nothing now.
I didn't think of the pee pads. But I'm thinking they will have even more fun with the cotton like material in those. LOL