My son and it seems most kids this age and older love finding cool rocks (minerals). He could explain some physical properties of minerals. Easy stuff some are hard and some shiny. It seems where ever you are you can find a cool rock and it is fascinating.

My son did this about this age. We found some geode rocks he took as examples. Bust them open and see whats inside.

http://www.kidsloverocks.com/html/physical_properties_of_mineral.html