If you are painting the barn, for example, ask the class to come over and bring an old can of paint from home. Take pictures of the barn in all of its glory before you paint it in the next week or two. Don't use too much frosting. Healthy can be good. Kids don't HAVE to have the traditional sweets.
Cheese is also popular here.
Don't be afraid to ask for another parent to help, but make sure that you do so in advance.