Give each child a piece of duct tape tacked to a piece of cardboard with the sticky side exposed to the elements. When the kids see something interesting, let them stick it to the duct tape. It keeps kids looking for stuff to put on the tape and makes for a good conversation starter later. Having said this, beware that one of the rules of the trail is to leave things as they are. Still, I can’t see any harm in pulling up a single blade of grass or picking an acorn off the trail if the end result is more children growing up to love and respect nature. The key is to not start uprooting whole plants or torturing small animals (like lizards) by sticking them to the tape. Materials: Duct tape. Ages 3 and up.
Read more about day hiking with children in my guidebook Hikes with Tykes.