4th day free @ Budget + 1000 Fly Buys + 100,000 fly buys entry + cashback
I like it when you can stack incentives up to make something better. The 4th day free from Budget is a fairly standard offer, but then if you take a class D car (mitsubishi lancer or the like) they've also got 1000 fly buys on offer and will enter you into a competition to win 100,000 fly buys.
Add to that that you will get a bit of a rebate here in the form of cashback and its not too bad an offer.
I've linked this to the main Budget deals page that has both deals on it. Make sure you read the locations and the process for both offers. The offer is valid for rentals between the 5th of Jan and the 15th of March.