I have two simple games I play for this.
The first game is the Baskin Robbin’s Game, if you don't know it, they probably do.
You start from 1 and you say 1, 2 or 3 numbers until one of you must say the target number (31) and you lose.
*Normally this game is played to 31 and doing shots of soju. We don’t need to practice numbers above 20 so we just stop there, we also skip the drinking.
The second game is the Even or Odd Game, again, they may know ti.
Just say 짝수 or 홀수 if you don't want to explain. One person guesses even or odd. Then at the count of three, both of them hold up between 0 and 10 fingers. Count them up, if they guessed right, they get their point. If it is 0, they just get a point.