I love your idea! So much better than candy and they'll remember these experiences
perhaps you can get some ideas from these packs our library system puts together
State Park Kit
Metal detecting
or these kits from the Virginia Aquarium
This is the rock field guide I used with my kids.
When my kids were little, I made up plaster of Paris and stuck it in a mold with various things in it. Then gave my kids little chisels and brushes so that they could excavate them.