I took this trip a few years ago...It was during the time I was doing my Master's degree in Paris, and I went there during a long weekend. I spent about 3 to 4 days.
All I can say is that this city is so clean!! (at least it was when I went there).
It's really easy to get around too. I strongly advise to walk if you really want to see all what that city has to offer (yeah I know, me and walking!).
I liked the fact that the streets were small and built on hills. so as you go up the hills, you could take some nice pictures too!