Party activities for 3/4 year olds

My boy is turning four soon, and we are having a birthday party for him. (We weren't planning on having a party but he's made clear he both wants and expects one, and indeed uses it as a threat when he's angry with us! "You're not coming to my birthday party!")

Does anyone have good activities or tips for 4 year old birthday parties? We'll be inviting about ten or so kids from his nursery although I've of course heard some horror stories here on attendance - but his two closest friends should (should) be coming.