Scavenger hunts work great to engage kids when out in nature.
Split a blank sheet of paper into eight squares, four on top and four on the bottom. In the top four squares, write the name of an item they can find along the trail, such as a pine cone or a fern. For older children you can be more specific, such a bracken fern versus sword fern. Leave the bottom four squares empty. Since we should never take nature with us, have the kids draw a picture when they find the item they are looking for. For an added lesson, write "garbage" into the last square and have the kids carry a piece of garbage out with them.
Make sure you have crayons or a pencil for the kids to draw with. And don't forget prizes, one for everyone.