The bus looked like it was almost empty!
That was because we boarded it late in the afternoon. I would say the best times to board a tour bus would be between 11 AM and noon after the normal morning tourists have already gone on their tour. BTW, our tour which should have lasted only an hour went on for almost 4 hours due to heavy rush hour traffic. Also you can get off the tour bus at any stop and walk around and then later board another bus from the same company to continue the tour.
In Spain, when we travel there next year, we plan on using the tour bus method to see the various cities such as Madrid, Barcelona, etc....