The best travel tip for taking photos with few people at the more popular tourist spots is to go just before closing time. Wait until you are the last one to walk out and take you last, wide-angle,interior photo, just before the guard throws you out. I never had that luxury in Europe, traveling with a tour group. We seemed to hit everything at peak tour times, so the crowds were ever present. You will always have the people that will linger out front, but that can be a good thing, as it sometimes can give buildings scale.
This is St. Paul’s Cathedral in London, on a typical, dreary evening in London. It was late in the day, but a service was finishing up, so photos inside were not possible.