Corinne - Kent's advice is good - ask about the worst case scenario for your insurance. Make three calls - your own insurance agent, your car rental company, and your credit card company. Be horrifyingly explicit - what happens if I crash the car, it bursts into flames and kills someone? You may find that such liability is already covered in the rental charge as a matter of law. But damages to the car itself are your concern. I've felt much better about my options after those three phone calls. But I do them before every trip, because coverage changes, especially with credit cards these days.
Museum passes can be bought at the museums themselves and at Paris tourism offices. No big deal. Just buy when you're ready.
As has been mentioned, an IDP is not necessary in France as it is in Italy. However, when you're out in the sticks and you're pulled over for speeding (which is increasingly likely because of radar), the police will write a ticket and fine you on the spot. You don't want them struggling to figure out your state drivers license to determine which is your birthday and which is the expiration date. I've said before, you want a happy gendarme.
And, finally, Perry's advice is a nuisance on this site. Tom's Guide to Paris is certainly charming and helpful but it does not cover the details people ask about here. Tom would be the first to tell you that but Perry persists in making referral to that website as the catch-all response to any question concerning Paris. Pay him no mind.