For the flower girl she could toss leaves instead of flower petals.
For favors - little jars of maple syrup or apple cider or candied apples / kits.
Decorations - use an outdoor arbor decorated with leaves and lights. The arbor can then be installed in their backyard or yours. For tables, ribbons colors. I would use either a mix of burgandy, orange and purple, or if the area is already very colorful, an off-white cream and gold.
For centerpieces, you could use purple cabbage surrounded by other smaller flowers. For my centerpieces, I used triffle bowls filled with a small raised votive surrounded by popouri. Guests loved it, and the ladies fought over who got to take one home. Its been 10y, and as far as I know, they are all still being used!
If you are doing wedding favors, only prepare them for half of the invited guests. Men usually don't do wedding favors. My cousin had a wedding list of 200+. Everyone got 3 wedding favors - a leaf shaped cookie cutter, fancy candied apple, and something called an ice cream bomb (BIG scoop of ice cream dipped in chocolate). Almost half of them were left sitting on the tables.
Another idea instead of wedding favors - if you are having a bar for drinks, just do the personalized napkins. Have them be used for the drinks/hordourves.
Congrats.
M.