The following list is in no particular order (mostly alphabetical, but somehow toilets made it to the front!)
Most establishments do not appreciate a nonpaying customer using their toilets. However, MacDonald's is good for a pit stop if nothing else.
An extremely good investment is the purchase of a Museum Pass. It lets you into most of the museums and can save a considerable amount of money. But the money savings is not the most important feature of the pass – highest on my list is that you don't have to wait in long lines to purchase your entrance ticket. There can be long waits to get into the popular museums such as the Louvre, but with a pass you can just walk in with perhaps a small wait for a security check. Another feature of the pass, you don't feel guilty about visiting a museum for a short time. While in Paris for 2nd visit (only 5 nights), I visited the Louvre 3 times and the d'Orsay 3 times. After 2 or 3 hours my eyes would usually glaze over and then it was time to do a different type of activity.
There are different lengths of time you can purchase the passes for, and they can be purchased at any museum that honors them. Thus the best way to purchase them is to go to a less popular museum and purchase one (I got mine at the Rodin Museum ) instead of waiting in a long line at the Louvre.
If you are as fanatical as I am, you might be interested to know that the major museums are usually open one night a week, and the nights are staggered so you can chose a different museum every night.
The metro is a great way to get around Paris . That plus walking got me anyplace I wanted to go. The metro pass also is good for buses, but the metro was more convenient. I don't recall whether or not I got my pass before the trip. That might give the best buy. If so, your travel agent has probably already taken care of that detail. If not, you can certainly purchase one over there.
You will probably note that the average French person does not talk much on the metro. So a good way to signal that you are a tourist is to have a loud conversation with your buddies.
While riding the metro, notice how much your fellow passengers talk – hardly at all! Talking loud on the metro will call attention to yourself as a noisy American, and perhaps even attract pickpockets.
I did get my pocket picked while I was there, and the wallet was in an inside jacket pocket which might have even been zipped! It was my stupidity and I learned a lot from it. The real stupid thing I did in Paris was to carry my Metro Pass inside my wallet, which means I signaled the location of my wallet every time I got the pass out and then again when I put the pass back. It is much better to keep the pass in a separate place. About the pickpocket thing, I was lucky since the wallet was just thrown on the metro car's floor and I recovered it with no loss of credit cards. Also, my passport was in the same pocket as the wallet, and fortunately that was not stolen. I have since concluded it is safer to keep the passport in the hotel room than on my person.
I would get no Euros ahead of time since you will probably have to pay a service charge. ATM's are ubiquitous in Paris , especially at the airport. Similarly, I would keep any remaining Euros at the end of the vacation since hopefully you will be returning to Europe some day. Consider this your chance to make a play on the exchange rates!
I'm sure you know of those funny plugs, but most modern hotels have at least one outlet for a US electric razor. I found that a handy way to recharge my portable camera (did you know you can recharge those “nonrecharagable” batteries just by plugging your power converter into the camera? It at least works for mine!). Also, most hotels supply hair dryers. And if necessary, you can probably get a converter at the front desk.
Almost all museums in Paris (unlike London ) let you take pictures of anything you want. Special exhibits usually forbid cameras, and you should not use flashes when taking an museum pictures. Not only does this annoy other visitors, it is usually forbidden.