If you can spring for more than a bench, I suggest you think of something more sensory, something that shows movement and/or sound. Rusty Keeler is a member of this CN&N Ning site, and on his own website, where he sells a book for preschool centers, he has some sample pages that might be helpful. With some ingenuity on your parents' & staff's part, I bet they could pull together something great, that's not too laborious. (I'm assuming you've already got an herb garden outside, yes?) - I'll be really interested to see what you come up with, and how it goes. Please fill me in - sounds very exciting!
Last year we worked on a big mosaic mural with our preschool children which is going to be hung in the entrance to our new library. We asked parents to donate any tiles they had at home and the kids had a ball smashing them (wrapped up in layers of material first!) with a hammer to create little mosaic tiles. We had a big piece of board to glue the pieces onto, and a simple organic design that included our school name. We left it out for weeks and kids and parents could add to it when the spirit moved them!
I also love to look at the group art projects Teacher Tom does with his kids at Seattle West Community Preschool - he has the outlook that "Together we are a genius". You could take a look at the things he does with the kids here