90% of our grocery shopping is at Costco.
Pet food
Vitamins and meds
Gift Cards (we save a bundle!)
clothing
TVs
gloves, socks, Mixers, batteries
What do we NOT buy at Costco?
Costco isn't always the cheapest, but the quality of almost everything they sell is good. Their "instant coupon" specials seem to be based on the manufacturer chipping in. I don't believe that they every have any "loss leaders" like regular retail stores do to get people in the door, but some of the things they have don't make much money (like the food court).
I don't buy eggs at Costco any more. My local ones used to have two dozen standard white eggs pretty cheap. But now they seem to only have cage free and organic brown eggs as two dozen. I think I'd have to get a flat of five dozen, and we don't go through eggs fast enough for that. My Trader Joe's has basic white eggs cheaper by the dozen. I don't know if it has anything to do with California laws that don't allow eggs to be sold by "battery hens".
I like the bananas they sell. Sold by the bag with a nominal 3 lb weight. I prefer the smaller ones as opposed to other retailers that sell them individually or perhaps weighed.
I'm willing to get almost anything there if I need it, but I'll look at the price and figure out if it's worth it - especially the monster sizes.
I've also gotten car batteries (their warranty service is excellent) and tires. When you get it there you can get free rotations and rebalancing for the life of the tires. Not all tire shops rebalance for free.
One of the more interesting concepts they have are "Costco Business Centers". They have some of the items that regular Costco stores have, but they have stuff like restaurant supplies, office supplies, and resale packages - and they will sell to any Costco member. There's one on the way to/from work and I've gotten snacks like single-serving bags of Cheez-Its, pretzels, cookies, etc. I got a case of shot glasses, stainless steel bowls, measuring spoons, tongs, etc. The first time I saw one of these was when I was in Las Vegas and my wife needed to adjust her glasses. I just punched in Costco to my phone mapping app and it sent me there where I found out it wasn't a regular Costco and the optical department went away when they changed the format.