For very short waits
Tic-Tac-Toe and Nim can be over in a couple of minutes. SOS lasts a little longer but is still easy to stop and restart. If there are three or more of you, Categories turns the waiting itself into a timer.
For children
Keep the rules light. Let younger players invent clues in Guess the Word, or use Categories without scoring. The aim here is not to turn waiting into homework. It is to give everyone something small and shared to do.