Why Leaves Change Color  is a great resource for around 3rd grade and up. It explains in Q & A format why and how leaves change colors.
Biology for Kids  also has a page about photosynthesis, along with other activities about plants.
The US Parks  has a page where you can find out which colors are dominant in your state and when you are most likely to see them in the fall.
The NeoK12 site  also has a lot of videos, quizzes, and other fun information on photosynthesis.
Learn about the rotation of the earth from EarthSky . This website has real-life pictures of how sunlight falls on the earth during the spring & autumn equinoxes and solstices. If you scroll down the page linked above, you will find videos taken from space & put on YouTube  by NASA.
Geography and Science
Learner.org  has Journey North . This program introduces the three sets of investigations that are featured in the Journey North program: Seasonal Migrations, Plants and the Seasons, and Sunlight and the Seasons. This is an interactive website where students observe nature’s patters and report it to the database. Right now, students are tracking monarch butterflies and hummingbirds in their winter migration. Students continue to check this site until spring, when they watch the migration change direction to the north.
Agriculture and Math
Find out what your local harvest foods  are. Once you know, you can find recipes for your favorite foods!
SeasonalRecipes.com  will also tell you what is in season as well as provide recipes for using those items. Fall is a great time for teaching real-life math in the store (budgeting, couponing, etc.) and through baking!! My kids always get excited about the days that we do math in the kitchen. One of our family’s favorite fall recipes is Gluten Free Pumpkin Muffins . It uses canned pumpkin, but if you wanted to use a store-bought pie pumpkin, (get the smaller ones, not the ones used for Jack-o-Lanterns) you could make it into a memorable teaching unit. Here is a guide for how to cook a fresh pumpkin .
There are several autumn-themed math activities over at MathWire  too!
Here are a few books that are great for the autumn theme and typically available to check out for FREE at your local library!
Here are the links to the books on Amazon so that you can read the reviews for each one…
Top Secret  – John Reynolds Gardiner
How Many Seeds in a Pumpkin  – Margaret McNamara
From Seed to Pumpkin  – Wendy Pfeffer
Johnny Appleseed  – Reeve Lindbergh