My favorite Costco is in Iceland: a place where things are normally absurdly expensive, and Costco Iceland is merely somewhat expensive. Arrive, stock up on food for a week (to supplement with restaurants, but some savings). Costco Japan would be really fun to visit too (haven't yet).
Also Costco + Internet (Amazon, B+H, etc.) are essentially 99% of my shopping here in Puerto Rico.
Visiting Costco, Walmart, etc. immediately after returning from war zones with limited supplies is a pretty shocking experience; I can only begin to imagine what it's like for someone new to the country who is actually from such a place.