IMO, some of the best places to proof a dog are ballparks and playgrounds. Not necessarily right in the fracas, but starting at a distance and moving closer and closer. Ballparks typically have multiple ages of people making all sorts of noises, food, helmets, bats, faceguards. kids running, etc.
When training for TDI, we use all sorts of hats, umbrellas, people eating food, wheelchairs and walkers.