We lived in Italy for 2 years and traveled all over, including a few times to various parts of France (Paris and other regions).
1) Carseat. I would get a cosco scenera. Be aware that even if you buy a seat they might make you take baby out for take off and landing because international carriers on international flights do not have to follow FAA rules. It's a PITA but that's the way it is and no matter how you fight they will not budge on it. We used Marathons in our home car but on trips we did the scenera for DS (18months-3.5 years at the time) and it worked great and is super lightweight. Installs well in every car we tried (FF, but I can install it RF without a problem as well, it's just not as easy- practice in your car beforehand). Any rental car will have LATCH-called isofix over there.
2)We hade 2 kids so we travelled with a phil and ted's over there. It was heavy but so worth it. The streets are no joke- so bumpy and uneven. We had no problem with the phil and ted's fitting wherever we took it. Maybe get something like the mountain buggy swift? It's very compact and lightweight but has air tires. It really will make a big difference in push and comfort. You can get a carrier too like the ergo and have it with you. Bring a bike lock for the stroller and stow the ergo in the basket so in case you want to go somewhere (a church or smaller museum) where they don't allow strollers you can lock it up for your trip inside. DH and I usually just took turns going into churches since the kids didn't care about that stuff .
3) We travelled all over in some more rural areas and every hotel, inn, and VRBO we stayed at had a pack 'n play. They call it a "baby cot" or simply "cot" over there. They are much bigger- mattress is same size as a regular crib mattress over here. Nice!
Hope that helps. Where in France are you going to be traveling?
M, mommy to A 2005, E 2007, and L 2010