Yeah i left the inside of my beds unstained. Linseed oil maybe a good alternative, I just don't see any of these products lasting long though.
Possibly Cedar or the fake wood. The new treated wood is so lame now it doesn't hold up. Maybe a couple years longer than plain old pine. Cedar and the fake wood are expensive. I could not find 12 inch widths or 2 inch thickness like on pine. You would have reinforce them, and they too will eventually rot.
Stone pavers would last, but they do move, and again are expensive.
My beds...
Same beds two months later 2013 08 15