The high lead and chemical content of most gun stores might make opening a food section very difficult legally.
There's a gunstore in Tulsa, Oklahoma that has a deli in the back. The sandwhiches from the deli aren't any better than the service in the gun store but I have eaten there and I have bought a gun there - neither was great and neither was really bad either.
The other things I want in a gun shop is just to be there. Follow the rules and the laws to the t so that you will be there when I want something.
Provide fair prices. You don't have to beat the online shops but you shouldn't be uncaring about the costs of buying a gun and the sacrifices I make when buying from you. Make some money but leave me some, too.
You don't have to carry everything or have what I want in stock. Have a couple in expensive high-end options in handguns and rifles and have a couple low end guns in stock, too. That way if I really want to carry something home today, I have a choice. If you don't have what I want, no problem. I know you'd order it for me with a smile.
If I just feel the need to talk about guns sometimes, let me hang out; talk to me. If you're too busy, I'll understand if you tell me when your slow day is or even simply say, "Hey, I'm pretty busy right now. Browse for a while and I'll talk to you when things slow down." Heck, maybe I'll spend money while I wait. Or maybe I won't have time to wait and I'll come back when we both have more time.
When I ask about a product, feel free to give me your advice and even to suggest the product you have in stock over something I may want to order - if the in-stock product will do the job for me just as well. If, after your suggestion, I still want the product I had in mind when I came in the door, then order it for me.
If you're losing money because you have to pay up to 3.5% surcharge when I pay with my credit card, don't be afraid to tell me. I'll pay the 3.5% or I'll write you a check (which you won't mind taking because we'll have a long history of business together) or I'll stop by the bank on my way in and get some cash.
If you do all those things in your store, you'll be very much like the store I frequent in the small town in which I live, and you'll be busy, and you'll be successful.