One deck, half an hour, and a skill they keep for life.
What you need
a deck of playing cards
How to do it
Pick a real game with simple rules: Go Fish, Rummy, Whist or Cheat.
Play the first hand with all cards face up while you explain.
Play a proper hand and let them lose it without softening the outcome.
Talk about one decision they could have made differently.
Play again straight away rather than stopping on a loss.
Most activity lists ignore the things that actually decide it
Card games involve probability, memory and reading other people, all at once and without any of it being announced as learning. Playing again immediately after losing is the part that builds tolerance for it.
Adapting by age
4 to 5 yearsSnap or Pairs, played face up.
11 to 12 yearsIntroduce a game with bidding or bluffing, where reading people matters.