Cardboard box upside down so like a den - two holes cut in it (entrance and exit) and filled with scrunchy brown packaging paper (that you get in every courier delivery these days) :wave:
My two LOVE this in their hutch - bang around in that box for hours doing god knows what! :roll::lol: Two days later the paper is all ripped to shreds :lol:
My two had an epic Xmas because of this stuff! Obv be careful they're not eating the stuff. George had a fabulous time tearing this to pieces.
Also like Sky said, tubes with hay stuffed either end and something in the middle - they went mad for it.