Let the birthday girl or boy pick a few crafts and games (safe and appropriate for your target age group, like decorating cookies or making music with tambourines), and set up a kiddie space in the den. Cover any surfaces you don't want ruined. Rotating through activities will extend attention (and enjoyment) levels, and the little ones will go home with party favors they made themselves.
A craft or party store will have all the supplies you need, and the overall cost should be quite reasonable. See if a grandparent or aunt might help run a station so you can be free to run the show.
Up next: Bring in a pro.