A fun way to do this is to set your child up with an audio recorder: the old cassette version can be found in second hand shops for only a few dollars. Bring lots of blank tapes and a set of headphones.
Articulating on tape everything they are noticing keeps them engaged in even the most mundane travel activities. Sightseeing takes an interesting turn when the child is encouraged to capture the moment. Preschoolers through elementary school age children love this — and so do worn out parents.
Bonus: having your child’s young voice on tape is a priceless treasure.