We always do a park party... its cheap and you can spend more money on the food and decorations and games. Best thing about it is that you can have different aged kids and they all have fun as it is in a park and the older ones can kick a footy or play cricket etc.
Still have games like pinata, pass the parcel etc but we find that them all just playing in the park is enough and they are not interested in the games!
Weather is the only problem though...

Got your PM, sorry i didnt reply all GREAT here thanks, when are you at work next?