2 in the household makes bulk buying not so necessary. However, we do stock up on:
coffee, certain herbs and spices, canned tomatoes, V8, meats like vacpac corned beef, rack of lamb, sausages, and some of the cheeses, omg and the peanuts, as well as a few non food items. Our Costco also has a liquor store and there are some great prices there too.
BJs is another with several different lines of products that we get once in a while. All in all we save enough to justify a membership there too.