H.K.
We had a similar situation once and had to quickly turn an outside party into an inside party...
We broke out the dress up clothes and clown makeup and let the older kids dress up the smaller ones and put on a fashion show, even the adults got into it, letting their kids put clown makeup all over their faces.
You could turn it into a cooking party with some quick kid friendly recipes and let everyone cook...
You could let the kids make home-made play-doh and roll it out and play with it...
You could make a scrapbooking party...take digital photos of everyone, print them out quickly and let the kids make themselves into funny creatures with yarn hair and google eyes etc...
Hope you come up with something! Good luck!