Activities to try:
identify and take pictures,do bark rubbings, collect leaf samples,measure the diameter etc.. of the trees in your yard or at your favorite park etc..
*look up what your State tree is and find out why
*Difference between deciduous, coniferous trees etc...
*Food products we get from trees
* eat some apple smoked bacon or other meat and talk about the processof using wood for smoking meats
*Ways to identify what kind of tree your looking at i.e. leafshape/size, overall tree shape, bark characteristics, seed type
*make pancakes and eat with maple syrup and learn about the harvesting and making of maple syrup
* learn how charcoal is related to trees ( since it is summer bbq time)
*learn why trees are essential to our ecosystems
*learn about how paper is made and make some paper
* learn about photosynthesis
* diagram and label the parts of a tree ie roots, branch, bark
* study deforestation and how it is effecting parts of the world
*learn how to climb a tree safely
* learn about growth rings on trees
*learn where there are old growth forests left in the US
* learn about all the uses for trees especially hidden uses like rubber trees, medicinal uses, amber for jewelry,cinnamon bark, cork,pitch etc..
* what are the tallest trees in the world, fastest growing, which havethe biggest seeds, desert trees, hardest wood in the world
* learn about petrified forests
* learn about the scale of rating wood for it's hardness and the scale used to rate wood for it's burning potential

This website by the National Arbor Day Foundation has a wealth of teaching materials. www.arborday.org