My fiance and I basically eat lunch at Costco for free sometimes due to the samples they give out. On a Saturday or Sunday afternoon, depending on what they're giving out, we literally don't eat any additional lunch that day. I'm turning up my nose at the hot dogs, but considering some of the samples they give out are highly processed meat and other processed stuff, I'm sure our "meal" is of similar quality to the hot dogs.
Definitely agree on not buying treats there. It would be sooooo easy to buy one of those industrial-sized bag of dark chocolate covered pomegranate treats. (Cue the slobber.) We just don't. The only time we've ever even bought something from the bakery was to have an apple pie for a Thanksgiving crowd of fourteen people. Their bakery is amaaaaazing.
We buy meat only infrequently now (really only if we're cooking for guests), but when we do, we look pretty carefully before buying there. For one thing, you're buying a lot of meat. Usually we don't want that much. Also, there are definitely times when Kroger is cheaper for comparable quality, so there's no point in buying Costco quantities and freezing the remainder.
We do buy paper towels, TP, cat litter, canned tuna, honey, rice, and other such stuff there. Oh, and big blocks of cheese. And the peanut oil we use for frying our Thanksgiving turkeys! So much cheaper at Costco! Plus there's enough left over to use for regular cooking for quite a while. Oh, and cases of beer, usually in the form of a sampler. We've really slowed down on our already moderate alcohol habits though, so we don't get very much beer any more. When we wanted cheap wine for mulled wine at Thanksgiving for a crowd, though, we went to Trader Joe's for the Three-Buck Chuck.
I have been known to throw in something I don't need, like those so-soft-and-cuddly thick socks, but it doesn't happen very much, and I really enjoy those socks. His only weakness is big boxes of cereal, but a mild side-eye gets him to make the right choice and put those back. We have been known to make a dumb decision and end up with months' worth of juice boxes after our nephews visited. Now I have a 54-year-old and a 28-year-old sucking on juice boxes so they don't get wasted.
I recently got my fiance to start getting his printer cartridges refilled there instead of buying new ones. Better for his business's wallet and better for the environment. We've both gotten tires there as well.
Finally, we buy bags of fruit there. My fiance will eat a few of those easy-peel clementines every day.