You did not mention the details of your radio contract. Is it just Fridays? How many days per week? How many spots per day? How many stations?
I disagree with some of the comments above. Radio is a great place to put your money… if you are clear on what you want to accomplish and realistic on what radio is good at and are willing to put in the required investment. Many people are confused about what marketing is for and what different media can do. Radio is excellent at building name recognition and creating an identity and personality for your business… but it is a long term proposition. Radio sucks for an immediate call to action like the event you are holding or a special offer good only tonight. (Possible exception if you are investing in a really concentrated event like a live remote where you might drop $1000 in an afternoon and people already know your name)
One thing that is important to note is that we are in a small town in a rural part of the state and nearly everyone that can hear the signal is inside our delivery area… so the dollars spent are directed at our potential audience. In a larger place where the majority of the people who hear the station are not in your area the math does not work so well.
As part of our regular advertising plan we run 4-8 spots per day 3-5 days per week on four stations varying by season. Our radio ads never mention a special offer. They are all about creating an identity and talking about what makes us different. Sometimes the message about differentiation is direct, sometimes it is implied.
Putting a handful of spots on a single station one day of the week to promote an offer is a waste of money. This is especially true if many of the radio audience are not in your area. If you want to spend money to directly generate sales use coupons delivered by any of several means as several mentioned above… the danger is that businesses who only promote themselves with coupons and focus entirely on direct call to action messaging send out the message that price is the reason to buy and never build a brand that enhances the value of the product. I believe that you need to have both the call-to-action messages AND branding/positioning messages as a part of your overall marketing strategy.
We have done free pizza tasting with great success. When we do it, we make a big deal out of it and do it for one day for about 2 hours. We push it on facebook, we run 3 column by 6" newspaper ads for three days (two days before and day-of) and we do it right after school is out. Our target is moms with kids… so we do it from 4PM to 6Pm. Typically we get 300-400 people and go through about 100 14" pizzas. With the kids there we have to do a certain number of plain cheese or pepperoni pies, but our emphasis is on our higher end specialty pies because that is where we shine and are most different from the other places in town… so we go through maybe 20 “kid” pizzas and about 80 4-8 topping specialty pies. Total cost for food and advertising comes to about $1000 plus we run some extra labor cost. We set up tables outside our store and hand out slices and talk about the pizza. We allow seconds (and thirds)… this is all about people meeting us, trying our pie and feeling good. We do this any time we launch a new menu with new pizza choices. It ends up being nearly every year, but not quite every year.