We haven't decided yet but these are our top runners:

Plantable bookmarks- the paper has flower seeds in them and the bookmarks (postcards are also available) are also personalized at the bottom with our names and wedding date. From TheKnot.com, approx $1 each

There are plents of websites to choose from! You can get just the seedling with an option of many different kinds of trees starting at $1.50 each!

I really like these too! Seed packets and you get to design the colors and picture on your packets along with your names and wedding date. These are from AmericanBridal.com and they have a few different styles to choose from. They are
$1.18-$1.66 a pack depending on how many you order. I even found a website offering them at about 25 cents each, but they never sent me the sample pack I requested, grrr!
Anyone want to help me pick which one?!!