When I was a kid, we'd mix up plaster of paris and make casts of some of the tracks that we found. Might be fun for you and your grandson. If I recall correctly, we'd sometime spray the dirt/sand with hairspray first to set it and try to get cast to come out clean.
I did that once with my two kids. We took a hike and when we'd come across tracks I'd have then mix up the plaster and we'd pour it over the track and move on until we found another different track and then we'd cast it. After we'd found about a half dozen different tracks we had a bag lunch and then back-tracked over our trail and picked up all the now-hardened casts, took them home, rinsed off the dirt and then I had the kids look through a guide book to learn what animal made each track. Thirty years later the kids still talk about that lesson.
Steve
