Sometimes I'll use the time before a game to conduct essentially a mini training session. Sometimes it's to review a newish topic, and sometimes I'll introduce something brand new. I know for my club, at games is some of the only time we get on a regulation field. There's no point in wasting that time with stretching or half-speed passing exercises.
Tomorrow, my U12s have a scrimmage. Before training, we'll be working on circulating the ball out of the back (review) and goal kicks (new for some players). Doing certain things for a warmup right before a game makes them easier to recall, and allows you to get the spacing and other aspects contextually correct.