Take students on a walk outside to count the number of small, medium, and large trees in a given area. From this data they will then use the given information about human oxygen consumption and trees' carbon dioxide absorption to calculate the "worth" of one tree and the given group of trees!
This is a great "Earth Day" activity or just a nice activity to get your students outdoors and appreciating nature! You could also provide extensions for them to calculate the oxygen production of certain rainforests on Earth or to write about the importance of trees on our planet.
Thanks for looking!