We involved our US family: Before leaving we mailed each set of GCs road maps and they drew our route on the map. And we asked them questions:
1. What are the 3 major rivers that start in Yellowstone? Do you guys know? And they traced their routes.
2. Trace the Continental Divide.
3. Where does water flow on each side of the Continental Divide? And if you say the Atlantic and Pacific you'll only be partially right.
The kids had great fun and the parents helped with the research.
Just an idea. It did require that we send them regular emails as we traveled.