I put PVC tubes in my cage. When my girls were little they couldn't get any traction and climb up. I put a t-shirt inside the tube and tucked an edge of the shirt under each end of the tube to help secure it. It really helped. Within a few months they were big enough and strong enough that I didn't need the shirts for traction anymore.
My girls love to stick their heads inside the tubes and yahoo. It echos and seems louder. They will actually run to the tube to yahoo inside of it when we "talk".