Let the kids do the decorating. Cover a wall with craft paper and provide washable markers (make sure they don't bleed through the paper, or you may mark the walls) and crayons that the kids can use to write birthday messages. Also, remember to cover the floor if it isn't easy to wipe clean.
Keep with the do-it-yourself theme. A few great ideas are pizzas give each child a pizza shell that they can top any way they like; or sandwiches — give the children a choice of breads and fillings.
Rice Krispies* Treat:
Prepare a large quantity batch of the original recipe just before you plan on using it. Give each child a small bowl, spoon and pan. Fill each child's bowl with an individual portion size amount of the Rice Krispies* mixture. Let them make their own flavour. Have gummi candies, sprinkles, chocolate chips, etc. Once done, they can press their mixture into the pan to set. Provide cookie cutters they can use to make fun shapes to decorate.
Give each child a plain white t-shirt (they can be bought in bulk at outlets). Let them design and decorate their t-shirts. Provide fabric paint, beads, "jewels", and fabric markers.
It's a challenge to create a specific costume out of common household products like toilet paper and garbage bags. Then hold a fashion show to judge the creations. Award prizes for most creative, most realistic, etc.
Each kid gets to bring home their decorated t-shirt. If you like, you can also send them home with a goody bag filled with craft products like construction paper, markers, pipe cleaners and glitter.
Let the parents know the kids may get a little messy and recommend they bring an old shirt to cover their clothes. Have extras on hand.