Just a few thoughts regarding Paris. Not to sound paranoid by any means but in my experience Paris is one of the more dangerous European cities these days. There is quite a lot of street crime and violence, mostly gang related. Some neighborhoods are quite bad, especially around the highway ring and in the suburbs. Hard to compare to some of the ghettos in the US but you could still easily end up in trouble just passing by the wrong part of town. Some metro stations are also pretty bad at night. I've had more than one unpleasant experience at the Gare du Nord during my 10-day stay in Paris last year. Simply being a white person and walking in the street could attract hostility. At some point I was assaulted verbally, and almost physically, in broad daylight walking down a beautiful avenue not far from Bastille (another location where caution is advised because it's a place where a lot of political demonstrations/riots start).

I hope this doesn't discourage you from your visit as Paris is a beautiful city and the people are for the most part quite nice. Just keep in mind that the threat of theft or assault is real and it's a good idea to be aware of your surroundings. Though if you take some common sense precautions and try to blend in with the crowd as much as possible there is little to worry about.