If you took a poll of the most popular birthday food (aside from cake and ice cream), pizza would probably top the list. In that case, how about turning the pizza into the party? You can buy premade pizza dough at the grocery store, and some pizzerias will even sell their dough for a few bucks. To avoid the hassle of having to roll out the raw dough, you could also use flat breads as a crust. Grab enough cans of premade pizza sauce, cheese and whatever toppings your guests would like.
Chef hats and red and white checkered table clothes would also make a cute touch. Even if you can't shell out for the extras, be sure to have enough aprons or old T-shirts on hand for folks to wear. Tomato stains are a chore to wash out.
Young guests will definitely need help making their pies, but this is another good opportunity for parent-child interaction. Tweens and teens should be fine on their own as long as the toppings are precut, and adults put the pizzas in and take them out of the oven. While they bake, you'll have just enough time to open presents.