KMD The planks make a good substrate, if they weren't kiln dried you will need to screw them down well to the cross bracing to keep them from twisting. Now you put some plwood/metal/masonite on top and you have a solid/flat bench.
Soundguy You should move to Oregon, we get .10 per can for recycling (we have to pay .10 recycle fee when we purchase them) it's supposed to help keep the landscape cleaner.