I had a great tour with a woman from Paris Greeters; I said that I wanted to focus on architecture and history, and she took me on a 2+hour tour of the 15th arrondissement, highlighted some details on Hausmann-iesque buildings that I never would have noticed alone, took me to some fascinating sights (like a massive church made of concrete), and really knew her stuff overall. I learned a lot, and highly recommend it!