Card games, helping to cook meals (somemores!!!), picking berries, skipping stones, fishing, exploring the lake, stream, or beach, macrame projects, singing, telling stories, drawing, reading aloud.
Along the way they should be involved with navigation and trail skills. There is nothing like being in front of a big feature with the map in hand. "See where those lines are all packed together? See that cliff?" When-will-we-be-there is the perfect time to get out the map.
It's the perfect time for geology and other natural sciences. Field guides are heavy but might get mileage.
Coming around the bend in the trail to see a deer or rabbit for the first time with a child is nothing less than an honor.
Most of all kids need to play, explore, and imagine.
I like the photos idea. Add a magnifying glass or a pocket microscope. AND PLEASE LEAVE THE VIDEO GAMES IN THE CAR.