Help with a surprize party help?

Hey, im throwing my mate a surprize sweet sixteen, well i say surprize, the story is shes wanted a party for ages but her mum kept saying no coz money issues, so a couple of weeks ago me, my mate and a few other mates gpt together and sed about chipping in and throwing a party buh telling everyone else its a surprize otherwise everyone would go against the idea, buh because shes in on the whole thing, i wont to try and surprize her in some sort of way like at the party, wether its to do with decorations or what not, got any ideas?