Given that you cannot expect everyone to come back every week, even the diehards, since life happens, I would actually focus on small, bite-sized adventures, rather than a megadungeon or hex exploration, each of which can end up with player characters stranded in the middle of nowhere or, worse yet, halfway through an encounter they will never get to finish with the same group of players.
The
Five Room Dungeon model is your friend. (It doesn't have to be a dungeon, it doesn't have to be rooms.) The model has the benefit of being geared around player (not character) types, so that the person who wants combat should see some, the person who wants to solve a puzzle will get a chance to, the person who wants to roleplay while you do a funny voiced NPC can, etc.
I've found, generally speaking, a five room dungeon created with this model (as opposed to five rooms in a megadungeon or something) takes about two to three hours to complete, which feels like a pretty good average for a store game. If you find this fun and aren't exhausted by it, you could even schedule one session in the morning and a second after lunch, so that the hardcore folks can get the effective experience of playing all day, but the people who just want to drop in for a casual game can do that.
I would also suggest you start off with Heroes of the Borderlands as a warm-up, which will help familiarize you with the 5E rules and, frankly, which will also likely help you sell more than a few boxes as well.