Some advice for visiting Paris (or any European city really):
1. Try not to look like an obvious American - no white socks, no T-shirt with whatever printed on it, no short pants, no big honking camera around your neck (if possible), no tennis shoes, no ball cap (they call it a casket). You still will, but will be less obvious.
2. Carry a thin wallet with minimal money and one credit card in your front pocket, and if you want a fake wallet in your back. Have a few euros in your other front pocket loose for "incidentals". Don't pull your wallet out and leave it on a table at a restaurant. Don't let anyone take your credit card back somewhere, they will bring the charge gadget to your table.
3. If anyone stops you in the street, keep walking, they are distracting you while someone else steals your wallet. This will happen often, sometimes young girls asking if you speak English or lost this ring, whatever, keep walking. Parisians do not talk to strangers.
4. The Montmarte area is touristy and also pretty rife with crime and begging. Don't give money to beggars.
5. The buses work pretty well, you can buy tickets at any Tabac store. The subway works but is a bit dirty.
6. CDG airport is a mess. Allow a lot of time in summer to get through it. We almost missed a flight once and were 3 hours ahead of departure. In July and August, it is a sea of people and no lines you can find, just a mess. It can take a while to find which carousel has your luggage also. Ugh, it's a mess, I've never seen a worse airport layout.
7. If you rent a car, don't rent from Hertz. They will ding you for tiny dings. I've stood and waited on the wife several times and seen this play out with Americans. I rent from Enterprise now and they are fine. She rented from Hertz for her work, and always had insurance, and often needed it. They still hassled her about whatever. And be prepared in Paris for not being able to park anywhere.
8. Air France runs a bus from CDG into town which I recommend. It's called "Le Car" oddly enough. It used to be about 16 euros. It has 3-4 stops, one of which is near L'Etoile, which is near where we were staying at the time.
The Musee d'Orsay is a really nice art museum in my opinion. The Louvre is simply too large and everyone flocks to the famous paintings and ignores the rest. I guess the reason I'm disenchanted with Paris is that I've seen all I want to see there, and more, and prefer seeing somewhere new. And I hate the airport, but that's about the only way to get there unless you fly to London and take the train, which is rather interesting.
CDG is a mess, aptly named I think.