Soap boxes, toilet rolls - hung by a string and filled with hay, cardboard boxes. Old tea towels which you can peg to the bars and let pool onto the floor. Old bathroom mats.
Buy one or two of the very cheap doormats at Tesco's and wash them - they make good hammocks, hides to wiggle under and curl up on, and generally all purpose piggy lining. Soft washing up rags make nice pillows and can be dragged around.