The Geological Society of America has compiled a list of resources for K-12 Earth science educators. There is a wealth of knowledge on a variety of earth science topics from erosion to plate tectonics. If you had to start someplace, I recommend here.
One activity that is great for elementary kids is introducing them to plate tectonics and the layering of earth is to make a cake. I like to make three or four different kinds of cake: white, chocolate, rainbow chip and yellow. In putting the cake together, cut it into several pieces and jumble the layers alternating flavors. Then frost and decorate as you would normally. Slice your cake and begin the discussion on how on earth the cake could have ended up like that.