Theme party! Those are always fun no matter what age, and they dont have to be expensive. You have stores like Iparty that can get pricey but there are also stores like the dollar store (The Dollar Tree here in Maine). Lots of colors to choose from, and decorations. Also another great place to look would be online.
try www.orientaltrading.com They have tons of stuff at low prices. and the more you buy the cheaper the price. Check it out. And good luck!