Let's see ... old tires, fake christmas trees, old wood palletes, broken pieces of fence (with wood, make sure it's smoothed out - no splinters!) Maybe some stones if you wanna get a little crazy with it, barrels are great ... countless possibilities!! :)
Logs from old trees, make sure any limbs are at the very end. Dig down about 1-2 foot and fill with water about 2 -3 ft wide.. don't want too deep.
Old moldy hay bales or straw bales.. stacked up may want to pulll off the twine.