I like to go to the food court at my local shopping mall. It's pretty much empty during the day, has plenty of seating, different table heights, outlets, wi-fi, etc. Nobody will kick you out if you're not constantly sipping coffee, and you won't feel the pressure to keep buying stuff. As a bonus, there is a McDonalds often selling coffee for just $1. And when it's lunch time, well, there's a dozen restaurants to chose from!