Birthday Party Angst :)

Today, I’m asking for some advice from all of you experienced parents out there 🙂  Tomorrow, our C turns 4 years old (I can’t believe it!), and we’re having a birthday party for him in the late morning/early afternoon. My plan is to let the kids play for the first 30-45 minutes, and then to move on to pizza and birthday cake. Any thoughts on activities or games for the children, or just on what has worked for your family in making a birthday party enjoyable and fun (rather than stressful)?