Anything that can't be nailed down. Cans, jugs, old chairs and other objects, electonics, etc. Me and my friends constantly find things to shoot out in the woods, the owner treated the place as a junk yard for a while. WE have found old cars, bottles, barrels, wooden crates, GI Joes and alot of other old crap. Also try to add some difficulty to it. Like if your are a hill or gravel pit test how accurate and how fast you are by having a can roll down a hill and keep shooting it to make it roll back. Or try hitting a target from a moving vehicle, though I wouldn't personally suggest this unless you have a very controlled environment. There is a bunch of stuff that you could try to keep you interested. Even shooting at cans or things of that nature can get just as boring as shooting at paper after while.