This is one blogger's great list of her travel essentials, and her recommendations are right on track.
Having just returned from a two week trip to Idaho, Yellowstone and Grand Teton National Park, I'll add that my new travel essentials also include a small flashlight and a travel-size first aid kit. I knew we'd be doing a lot of hiking on our recent trip, so I brought along a small first aid kit. At the time I figured it may be excessive to bring, but when I was unexpectedly stung by a bee/bit by a fly (still don't know what happened), it was very helpful to have something to clean my arm and a bandaid to cover the spot...especially since the closest medical station was nearly 2 hours away.
We didn't bring a flashlight on our last trip, but from now on I will. It would have been helpful for night walks and cave exploring; even if you aren't hiking, having a flashlight in case of lights out in a hotel or other place would be useful.