This particular Panera is one of the best places I've found. The coffee is good and the refills are free, the breakfast pastries are tasty (I am addicted to the four-cheese souffle, and the cinnamon crunch bagels), and the people watching is excellent, too -- picture windows all around, and it faces a busy intersection with three train stops. So people and trains and cars are always flowing by. As long as I don't sit too close to the door (as I have today, unfortunately), I could hang out here for hours.
I haven't yet had much success with serious writing here, though. Whenever I come here, I end up surfing and/or writing on LJ. Then again, I haven't really tried to buckle down and work on fic in public much; if I'm in the right mood I bet it works really well. A holiday is coming up; maybe that would be a good time to check it out.