Chains spend a lot of time and money researching this topic so why not ride on their coattails. For example, Jack in the Box started a “Jacks Club”. You get a Plastic Membership Card that they swipe when you make a purchase. You have to regiister your membership so they have your email for further promotions. When you’ve spent $50 bucks you get a free meal. It’s a point for every buck you spend. They round it DOWN so you actually spend more than $50 bucks to get a free meal.
Now, I realize you don’t have the computer system to support this but a manual punch card works just the same.
Waffle House has a “Regulars Club” Sign up and get Email coupons (Designed to have you buy something along with free food) I’ve gotten 3 coupons so far and haven’t used them but they created top of mind awareness with the email. I’ve eaten thgere shortly after getting coupons and forgot to take them. Same with my Jacks Card. I get there, order, then discover I forgot my card. 
BTW, Jack in the Box is now operating “Munchie Mobile” FOOD TRUCKS!!!!! VERY Interesting!

 
i’m surprised the big chains haven’t used some form of mobile loyalty thing for you to get your coupons/points.. bummer that happens to me all the time when i get my boba here in mountain view.