By far and above the best ROI would come from database mailers. You have a customer database (assuming you have a POS, if not that should be your first priority), and you know 3 things: 1. Your customers like pizza, 2. They have enough money to order out, and 3. They eat dinner.
Door hanging is great for new customer generation, and that is a necessary part of your business, but in this economy, customer retention is key to your success. Go pick up a few Seth Godin books. Linchpin is awesome, but in your case:
Permission marketing: turning strangers into friends, and friends into customers.
and
Free Prize Inside!: The Next Big Marketing Idea.
Both of Kamron Karringtons books will help you in this department.
Back to direct mailers though, these will not only increase your customer purchase frequency, they will also increase your average ticket price. It is also easy to track your results, even if you have to do it manually by entering the returned post cards in an excel page.