Get some Mums! This is the time that most stores do away with the regular perennials and it becomes mum time. You'll be able to find pots of mums at the grocery store and home improvement stores. A few pots of mums go a long way in the porch decorating scheme of things. The mums can be an accent or a focal point.
Pumpkins and Gourds are the king of fall. They're also pretty cheap. Add some small pumpkins and a few large pumpkins for a splash of fall color. Put them at the base of your pots of mums and you've got a good start on a fall porch. Put them on the sides of your porch steps to direct folks to your front door.
A large porch can handle a couple of bales of hay. You can get three stack them up in a ragged pyramid with a top and two ledges on the side. Add your mums and pumpkins to your stack of hay and you've got a classic fall porch.