An extra leash too, just this past weekend we went camping w/ 2 out of 3 dogs & lost both leashes, what a pain. We were in aa tiny town & we spent 4 hours sat. afternoon going from one tiny store to the next no one had any leashes or rope, finnaly we found a tie down strap in the back of my truck & we used that for the rest of our trip, the fabric it's made of really bothered my hands. I did bring extra collars but forgot to bring extra leashes.
Like others mentioned, food, water, bowls, treats, toys, a bed & some basic medical supplies.