This is a great time to teach the difference between deciduous and evergreen trees. Basically, deciduous trees lose all their leaves for part of the year and evergreens are green all year long. This is not to say evergreens do not ever lose their "leaves", they just don't lose all of them at once.
Changing leaves also make great objects to practice classifying. Leaves can be classified according to shape, size, color, etc. You can also have one child write a description of a leaf and see if others can identify which leaf it is based just on their description. Afterwards, make leaf rubbings and use them to identify the main parts of the leaf: vein, blade, apex, base.
This is also a good time to discuss why leaves are green and why they change colors in the fall. Chlorophyll is made by leaves for energy. It is a naturally green substance and it gives leaves their green color. As the weather gets colder, the leaves stop making chlorophyll and the yellow and orange pigments are able to be seen.