baking things that go far but don't cost very much like Meringues which require 2 eggs and 100grams of sugar to make roughly 12 - 15 meringues. or jelly/jello - you can use fresh fruit juice and gelatine place in molds and they looks super cute. fresh fruit chopped up on kebab sticks. plating games instead of hiring entertainment! pass the parcel, scavenger hunt, dunking for apples etc... also do party packs made out of things you have - like recycling jars or containers that you can collect over a few weeks or months before the party then paint them or cover in scrap book paper or fabric. fill with a few sweets and a thank you :)
pick a theme where you don't have to buy branded merchandise... like mickey mouse or hello kitty. maybe rather pick a country (eg chinese) or a general theme like a rainbow party.