I started carrying a travel set about a decade ago, mostly as a joke. It has since been the single most useful thing I pack.
The pattern is always the same. I mention mah jongg, someone's face changes, and suddenly there is a table. A hotel lobby in Singapore. A community hall in Vancouver. A kitchen in Dublin with four people I had met that afternoon, using a card I had brought as a spare and haggling cheerfully over whose rules we were using.
What I did not expect is how much it changes the shape of a trip. Guidebooks give you the places. A mah jongg set gives you the people, and it does it in an afternoon rather than a week. You sit with strangers for three hours and learn things no tour would tell you — who is getting married, what the neighbourhood used to be, which restaurant is actually good.
The set is small, it is heavy, and it has cost me excess baggage fees in four countries. I have never once regretted packing it.
The rule I have landed on: if the trip is longer than a week, it goes in the bag. It has never once stayed in the suitcase.