For whatever reason, I end up spending what feels like a lot of time in the grocery store. It isn't a whole lot of time, actually. I can shop swiftly and efficiently, with a minimum of forgetting the exact item I visited the store for.
But, my very first job (other than the dog walking "business" I had when I was 10) was cashier (and later customer service) at a grocery store. Foodtown, if you must know, though it wasn't the Foodtown with the towering clown out in front so that's all right. But running a register can be inspiring in that "slice of life" literary kind of way. Also in a weirder kind of way, I guess.
Other experiences are from the customer side of things. For example, every time I've gone into Wal Mart in the past six months or so (which all told has been blessedly few) that Kary Perry song "Roar" has been on the radio. I cannot claim to be fond of that song. There are other times, though, when a song you LOVE comes on the radio. While I myself will likely never rock out to "Don't Stop Believin'" in the bread aisle, I can imagine somebody giving in to that desire.
Other times, you can tell when somebody's found what they were REALLY looking for, instead of a "sort of close enough" example; the grocery store I visit most has an organic section nestled in the middle of it, and occasionally I find those organic items next to the real thing. Like "Oh, this is where the rest of the yogurt is! I don't have to get this dairy free overpriced stuff made from coconut tears."
Obviously, fellow customers are also story fodder. That woman buying a shitload of organic, making-stuff-from-scratch food, using reusable grocery bags even when your store doesn't have a good way to do it, wearing a cardigan and driving a forty year old Volvo? There's something there.
The guy asleep on his feet buying a bajillion frozen dinners who gets a phone call from his wife, who is a night nurse? Yup.
Cell phones give us a pernicious window into peoples' private lives, because frequently people don't seem to understand that they are on the phone in public. That means everything they say is IN PUBLIC. Even when it's private phone-things to be said while you're alone at home or even in your car. It's interesting. And story fodder.