There are a lot of guides about how to travel. You should read them ... if you have the time. But reading and doing are not the same. So these are my rules - not fool proof, mind you - for a hassle free (almost) adventure. Guys - pay attention.
Cash: The best rate for switching dollars to Euros is still your local ATM. Most banks have them. Pull out 200 Euros or so at a time; your ATM vendor has an arrangement with a corresponding bank so you will get close to the interbank rate for the conversion, less 3% or so. You can also ask your bank how they will make the conversion - don't expect them to know. I have run test where I was sure to use an ATM and several credit cards, all on the same day - the ATM always has the best rate, but I can't see what it is from my statement. The total is just lower by a bit. There are ATM fees just as here - 1 to 2 Euro, typically.
Believe it or not, getting cash from an ATM when you arrive gives you a better exchange than if you went to your local bank before the trip ... big time. Local US banks don't deal in Euros. They will always have a correspondence bank - one of the bigger international banks or their corporate parent, maybe - but the interday rate they will use is usually 5% or more above the current rate ... the local teller will not know that. Ask what the rate is ... and then check the current "large volume" rate with Currency Converter. But if security is your thing, do the deal ... but not all of it. Just enough - 100 € or so to get you started.
ATMs, on the other hand, work througn big networks like Cirrus. Exchanges appear to be made reasonably close to the interday rates. Your local bank doesn't supply you Euros, they give Cirrus dollars. Cirrus gives the local European bank the Euro that comes rolling out the slot.
Cash is always necessary; so plan ahead - don't run out (because some restaurants don't take cards). There are very few major events where you actually need cash - but have some, anyway. The morning café and croissant usually call for cash. It's a lot less hassle.
Credit Cards: Most restaurants and hotels - even small ones accept Mastercard and Visa. Your credit card bank gives you a better exchange rate than the restaurant will, so take advantage. The toll roads - and there are many - also take credit cards. Plan ahead as you approach, because not all lanes accept credit cards ... and you don't want to try to back up. Gas stations will accept your card if there is an attendant, but the "pay at the pump" will not. The French cards have embedded circuits in them; the pumps will not recognize a card without it. Don't wait till after dark to try to fill up. American Express offers a "Blue for Business" card that has it. No Charge - few benefits except the IC. I just received one; I'll let you know if it worked.
I have used several cards as tests - most will charge an exchange fee of 2.5% or so. From my limited study, they appear to use reasonably favorable exchange rates, making their fee on the exchange charge.
Travelers Checks: We usually get some. We rarely cash them. Carol looks at it as found money when we return, but if you're friends with your bank, the Travelers checks are free, and it helps to have a backup plan. Most exchange places give you a slightly poorer rate of exchange - an extra fee for cashing the check - but it's a backup, so don't worry about it.
US Dollars: I usually travel with US dollars tucked away somewhere. There are some places that accept no cards, no Travelers Checks, and if you ignored my advice about planning ahead and getting Euros, US Dollars may be the only currency that's recognizable - can you imagine walking into a local MacDonalds and trying to pay with Euros or Pounds? The French aren't much different, but in a pinch - card got lost or stripe doesn't work - you should be able to find somebody - a street "Change" booth - that will take them. Rate will not be very good, but your trying to have a god time - make the trade and take my advice - plan ahead, next time.
Those are the basics. The key ... plan ahead. Know the exchange. You'll do just fine.