We've been Costco members for years, the bin liners are the item I see mentioned the most but they are fab! Big roll of black bin liners! The white, recycled ones are good for the kitchen bin as well.
You do need space, we put some items such as toilet and kitchen roll in the loft and bring a pack down as needed. They did sell out of toilet roll at the start of the pandemic which we'd never seen before!
Yes to freezing the muffins - they are huge, half a muffin is more than adequate but a whole one is nice, their croissants are lovely too.
Ciabatta bread in bulk that you bake (DD's favourite for school lunch when her brace is off!), pack of 12 plan Naan bread,.
Big pack of craisins (dried cranberries), wine (I like the Kirkland brand of prosecco), they do alcoholic advent calendars - saw a gin one in there last weekend and DH has had a beer one from there.
Batteries, especially when on offer can be quite cheap. Brita water filters.
Chicken portions (chicken breasts and thighs) in the chiller cabinets, pork - meat is quite nice there.
Gallon refill container of Carex handwash, we go through a lot of handwash.
Also a big yes to the wrapping paper with the grid on the back, does help terrible wrappers like me.
The decorations (Christmas/Halloween) and furniture is lovely but the furniture is quite large-scale - looks deceptively small in the warehouse and larger when you get it home!