This is a tough one, because people usually need more time to work through real math trauma. But my go-to is either 1-2 Nim, or, if I have access to blocks, Pattern Block Triangles.

1-2 Nim2. When demonstrating 1-2 Nim, narrate the game out loud, using mathematical language, and leaving empty space for students to chime in: "My opponent just took 2 leaving... [wait for students] 5 in the pile. Who has a advice for what I should do next?"mathforlove.com
Pattern Block TrianglesMath concepts: Addition, counting, geometry Equipment: Pattern blocks, scratch paper and pencil Common Core: K.CC.4, K.CC.5, K.OA.1, K.OA.2, 1.OA.1, 1.OA.2, , 1.OA.4, 1.OA.5, 1.G.2, 2.OA.1, MP1, MP2, MP6, MP7, MP8 How many blocks you can use to build a triangle from pattern blocks?mathforlove.com

1-2 Nim is perfect because it's so innocuous and anyone can begin playing it right away. But the moment you lose a game, you know something is up, and you're already drawn into thinking mathematically!