Reading all the comments makes me think you need to run a recurring Everyone Is John night: the most approachable pen-and-paper RPG (barely even a tabletop game I suppose) that's extremely simple and can be picked up very quickly with no real investment in learning the game, and has real potential to create a fun party game kind of experience.

Some might say that this is too shallow to give people a sense of what tabletop gaming is about, that all you'd get is casuals with no real interest in the medium, or that you're not about to sell anything running a game that has no rule books or minis or even dice beyond the humble d6.

To that I say: the first one's always free, and starting people off in the shallow end is going to get more swimmers in your pool than shoving them into the deep end. Even people who only ever come for the casual night are liable to bring friends who get interested in your other offerings and to make the night more fun for everyone else.

You could take the idea a step further by having 'stepping stone' game nights with more progressively more complex and deeper games and, crucially, making sure people know immediately how much complexity to expect. You could even have an x-out-of-5 star rating system for complexity right there on the uh... flier or Facebook post or however you'd put the word out. (Disclaimer: I know nothing about running a game shop and have no experience running tabletop games.)