Seed packets with a small tag attached would also be a good favor (although I am one who believes that favors aren't always necessary).
I second the tissue paper flower idea. It is a Mexican tradition of some sort to have tissue paper flowers at fiestas. My DNiece married a Texan (not of Mexican heritage, but I guess they are familiar with lots of Mexican traditions, due to their close proximity) and his Mom and Grandmother made lots of beautiful paper flowers for their rehearsal dinner and explained the tradition. Each of the ladies got to take a flower home. They were beautiful and festive.
A layer cake decorated with fresh flowers could be your dessert as well as the centerpiece of your buffet table.
Choose flowers that are less expensive (like carnations, daisies, pansies) and use LOTS of them. If you want to dress it up a bit, you could add just a few accent flowers like Stargazer lilies, gerbera daisies, or roses... but those will cost you much more per piece.
I just thought of a centerpiece I received at a baby shower. They had filled a square glass container with potting soil and grown grass in it (grass grows fast, you would have time before your party to do this). Then in the grass they inserted just a few gerbera daises (maybe 3)... the flowers were cut, not rooted. You could do this if you could find inexpensive glass containers (check the Goodwill or Salvation army) and use less expensive flowers. I had great luck finding clear glass containers at the GW for DD's most recent b.day party.
Anyhow, good luck...........................P