The most common mistake made when designing a plaza or square is to make it too big. If the square is not sized right for the number of people that will be using it, you end up with a dead space.Small, intimate and lively is better, and by a happy coincidence, cheaper. Kaching!
Plazas naturally become the center for economic activity. Economic activity draws people, in a self-sustaining circle. Make sure there is plenty of public seating, including tables. I think a basic public restroom on each square is a good idea.