What makes a "good route" really depends not just on the type of road, but on local conditions. In Boston, a bike fits on pretty much any non-controlled access road (there's probably an exception, but offhand I can't think of one). This is because the traffic is so dense during morning rush hour that you're really not holding anyone up, AND the drivers are used to sharing the road with bicycles.

Do stay away from the doors, and do stay off the sidewalks. Sidewalk riding is really only safe in situations where the "sidewalk" would better be named "non-road strip of pavement with zero pedestrians on it". Pedestrians and bicycles don't mix.