I honestly do not think you can improve upon mother nature. That's the beauty of getting married outdoors. You could dress up the "altar" spot with a ring of flowers arranged on the ground where you and your FI could stand inside of
it during the ceremony. Or sprinkled rose petals along the altar site. You could also get some potted trees, like ficus, and decorate them with battery operated twinkle lights.
Rent some seat cushions in your wedding color(s) for the guests to sit on or buy several second hand quilts and fold them into thirds and use them for the guests to sit upon. If you're at all crafty you could make some pillows or long mats
for your guests to sit upon.
If you just have to have some decoration, how about a bamboo or willow branch arch draped with fabric? Or ribbons? Or cystals? You could also do a monogrammed banner stetched between two tall shepherd hooks or poles with a
heavy base (think our door umbrella bases with a tall wooden pole screwed into it and then a fabric banner with rod pockets on each side covering the polls). Check out this bio for inspiration:
http://community.theknot.com/cs/ks/user/page.aspx?username=archbio
And check out the ideas posted in the Picnic Wedding and Backyard Wedding threads...there's many things that would work for your site:
http://www.projectwedding.com/post/list/picnic-weddings
http://www.projectwedding.com/post/list/backyard-wedding-2
Here's some more inspirations pics:











