This is a wooded outdoor activity, but if you're stuck inside or away from the trees, improvise by having leaf and limb samples for folks to use. Select as many different kinds of trees as possible in the area and label each one with a number on an index card. (Tie the index card around the tree.) Give each person a sheet with the names of the different trees that you have labeled. Send the folks out into the "forest" to match the trees and the names.