Food
Many schools have adopted low sugar / low salt party policies for parties and birthdays. My room parents of 2007-2008 gave me the great idea to theme the food along with the party. For example, we ate all red fruits and vegetables on Valentine's Day and all green vegetables on St. Patrick's Day. Before Memorial Day, you can use cookie cutters to make stars out of pineapple like in this picture. Try dipping fruit in yogurt and freezing it instead of using chocolate. You can also replace the blue strawberries with blueberries.
Check out this chart for other ideas!
Repurposing and recycling to make table decorations
For Earth Day, we used newspaper for table cloths. We used the flowers we had grown from seed that were planted in newspaper pulp pots we had made. Our table confetti was made from paper we had made and imbedded with seeds so that they could be planted later. Students made picture frames using recycled cardboard and puzzle pieces from boxes that were missing pieces.
(Note that these are not my pictures, I didn't think to take my own!)
Check out my website for lesson plans and websites for teacher and student use.