As someone who lives here, the most important thing I would stress is: bring a wide lens! If you don't have the CV 21/4, you should be able to pick it up before your trip; if not, consider grabbing a used Super Angulon at the Paris Leica store. This city is very close in on itself, and even more so in the more touristy areas. I would take a 21, a 35, and possibly a 50 if you want a third lens. To me, the combination of a 21 and a 35 in this city feels about like a 35 and a 50 in San Francisco, to give you an idea.
The weather is nice, with lots of sun, and the streets are well lit at night. 100 speed film is perfect during the day, but if you don't mind stopping down, you'll be fine with slightly over exposing tri-x, especially if you have a yellow filter attached. An advantage of the tightly packed buildings is they reflect light at night, so you can hand-hold night shots at EI 400 or 800.
