Ghirardelli Brownie mix. I’m a home baker who would never use a packet mix. Except this one. It is soooo good.
Kirkland is the Costco own-brand. Pretty much anything by them is good quality and good VFM.
The one exception IMO is the Chicago Best Jumbo Beef Frankfurters. (Actually, I’m not sure the ones in the cafe are Kirkland.)
The fresh chicken schnitzels. We always stock up on schnitzels and hotdogs and freeze them. Ditto for a side or two of fresh salmon (portion up, first) and a tray of burgers.
Before you go, it’s worth noting down the prices of things you like at the supermarket. We find that branded foods tend to cost about the sand at Costco as supermarket own-brands, and it’s easy to get carried away. It only saves you money if you are self-disciplined.
Don’t be afraid of buying high-value items. Costco are brilliant about returns. Again, have some idea about models and prices before you go, if there is something you know you are interested in. There is usually only one model of anything.
Check if the Costco you are going to has a petrol station - premium grade is often the same as, or cheaper than, standard grade elsewhere. Expect a massive queue.
Give yourselves time to have a bite at the cafe. Not exactly haute cuisine, but an experience - a tasty one.