I am not sure if you guys were as obsessed with playing "grocery store" as a child as I was - but I think it would make for such a fun party theme!
Clean off your hutch (and realize all the stuff you never use ;o) and turn it into a food stand! Scales and crates of food set the tone of a road side market! Source
You can build a checkout or farm fresh station out of cardboard! You could build one for each kind of food and set up a whole market! Source and tutorial
Setup dry good bins where the kids and scoop and bag! Little signs for names and prices finish it off! Source
Set up water, bread and fruit stations for the kids to "buy" for the lunch! Make sure to give them "money" to spend! Source
Don't forget fruits and veggies are BEAUTIFUL! Use them as decorations wherever you can! Source
Use little mini grocery carts (I have seen at World Market) to use as centerpieces! Source