If you want to add stuff right now, here are a few possibilities:
Cardboard boxes (until they get squashed or wet), more pallets, tree stumps or sections of log, bales of hay or straw, sawhorses, cinder blocks, large stones or piles of bricks (be sure they won't fall over), old tables or chairs (only if you never need them clean again).
Basically, as long as it won't hurt the chickens, and you don't mind having chicken poop on it, anything can work. Give serious thought to anything you would otherwise throw in the trash-- it might do some good in the chicken run first, and you can throw it away after you have the nicer things you want to build.