First of all, let me tell you a bit of a story. When I was in my 20’s, I had an apartment with a pizzeria on the corner. I was a regular customer there. The owner would, whenever he was trying something new, give his regulars a taste and asked them what they thought.
Whatever you decide to try, I recommend offering some of your regular customers a try (maybe attached to some freebie, like a quarter of a hamburger or something like that) and ask their opinions. You get good feedback and solidify your customers’ loyalty.
The ideas that people have given are fairly good. One thing you might try is simple horseradish and mayonnaise (I find that Kraft Miracle Whip and its clones make a good horseradish sauce when mixed with plain horseradish).
If you do that,also say to the customer,I seen you come in here before and I wanted your opinion,what do you thing of this?and give then the sample.You can say that to a lot of customers.why say it makes them feel like there in the know.so they tell their friends, and bring their friends,because they feel like someone there.