About Costco, the no-brainer for us was diapers. Since then (6 years ago), our Dominick's had begun carrying the monster pckg. at the same price, but we've moved past diapers and still find it worthwhile.
As discussed in other forums, the House Brand Extra-V. Olive Oil (both the plain variety in lg. jugs, and the vintage dated version in .750s is a huge bargain).
Ditto the smoked salmon. Not like specialty-store quality, but for an informal brunch or party, you can't beat it.
Various small appliances and cookware.
Occasional "finds" among the books/CDs/DVDs
House label utility clothing e.g. jeans, underwear, cheap gym shoes, occasional good-enough-for-the-casual-office bargains as well. Also kids' stuff.
Artichoke hearts, Calamata olives, Stacy's Pita Chips in the huge bag, rice crackers and snack mixes, breakfast cereals.
Butter, juice, canned goods, crackers.
Produce is iffy, but worth looking at.
Cheese (not "Stands Alone," T.I. or Caputo's level, but again, excellent value for money for Manchego, French Feta, Mozz., goat logs, etc.)
In general, lots of stuff which is marked up way past the insulting level at your avg. grocery, is tolerably priced at Costco.
The lefty bleeding heart in me sometimes worries about the meat: is it cheap because non-unionized workers with no benefits are being crippled by carpal tunnel syndrome while they slaughter hormone-addled animals in abusive, unsafe conditions? I don't know. But I do like Costco overall.
As far is entertainment derived from speculating on the contents of others' carts---This didn't happen at Costco, but still amused me:
Stopping into my local Dom's very late in terrible weather for some necessity, like diapers or cat food, I stood behind someone who was paying for the following:
* Economy size cooking oil
* Lg. role of duct tape
* Lg. pack of batteries
Nothing strange about any single item, but you have to wonder about the circumstance that would absolutely require you to go out for just those 3 things, in terrible weather in the middle of the night. At first I giggled, but giggles quickly turned to shudders as imagination ran rampant...
"Strange how potent cheap music is."