Carry some pesos before you walk off Quinta Avenida

On Quinta Avenida, dollars work and nobody will stop you from using them. So why bother with pesos? Because a few streets away, pesos are what people actually use, and you get a puzzled look when you hold out dollars for a coffee. I always change some money before I leave the pedestrian street. Coins and small notes are the useful ones, since a big note for a small purchase is the thing that slows every counter down.