One word: balloons!
And lots of them. Dot the walls with colorful construction-paper circles, and cover the table with a polka-dot tablecloth, plus matching plates, cups, napkins, and straws. (You can decorate everyday drinking cups with plain round pricing stickers from an office-supply store.) Before the guests roll home, hand out goody bags stuffed with a bottle of bubbles and a lollipop.
Rustle up some bubble wrap for tidy (if noisy) fun, or create a ball pit by filling an inflatable kiddie pool with lightweight plastic balls (try Fun Ballz, $20 for 100, amazon.com). A battery-operated bubble machine (like the Gazillion Typhoon Ultimate Bubble Machine, $25, toysrus.com) will save you from blowing a wand and keep the kids running around in circles. (Plus, it will help them burn off some energy after that doughnut cake.)