We’ve been going to Costco for years and number 1 tip is to know in advance what items usually cost - it isn’t always cheaper in bulk.
We usually buy meat (better quality than supermarket but not usually cheaper), toilet paper & kitchen towels, cleaning products, Comfort, Persil, Marmite & peanut butter, squash, cereal/porridge, cheddar cheese, butter, spreadable butter, Heinz baked beans, canned tuna, pasta, sometimes toothpaste packs, batteries.
They also do good variety of children’s book packs for all ages as well as adults books; I’ve bought Skechers shoes and DH has got Levi jeans. DC once had a fireman fancy dress costume from there which got countless compliments and asking where we got it from.
The cakes and made-to-order celebration cakes are great too. We’ve bought their ready made meals a few times ie pizzas, cottage pie and Mexican chicken wraps which are all good and freshly made. The platters and bakery platters are great for catering.