1 sheet of paper and pencil for every two girls
1 garbage bag
twice as many items as their are girls that DO NOT belong in nature (such as a napkin, a drink bottle, etc)
Place items around a hiking trail, making most of them non-obvious. Have the girls stay with their buddies and walk the trail. They are to write down the items they see without touching them or pointing them out to anyone else. At the end of the hike take the girls papers and lead them back down the trail pointing out the litter and have them pick it up and put it into the garbage bag. Once done give the girls back their papers and let them compare how much they found and how much they missed.