Western/Northern Europeans don't usually tip. Just round up to the nearest Euro. This applies to taxis and restaurants.
You'll be in Paris and Italy. For God's sake, dress up man! Don't dress like a typical fat American tourist. This means no baseball cap, no tennis shoes. Get some comfortable dress shoes. Blue jeans are okay. Nice shirt, sport coat. Europeans dress up much more than Americans.
Women: Be prepared to break your neck in either France or Italy.
Money: Best way to get your €uros is to use a ATM/debit card. You'll get the best exchange rate without exchange fees. Be sure to inform your bank that you'll be abroad. Find out if your US bank has any partnerships with French/Italian banks such that you are charge minimal or no ATM fees. (As a USAA card holder, I am not charged ATM fees in Europe.) Far fewer places accept credit cards in Europe but be sure to have a Visa/MC card handy. Right now, $100 USD buys you 77 EUR as of 9:52 EST. I would recommend withdrawing increments of 75 EUR and deduct $100 from your checking account each time.
Safety: Don't be a dork. Don't use a money belt. I travel with a money clip containing 100 EUR cash max along with my US driver's license and various other cards (ATM, credit, health insurance, etc.). I make photo copies of these and leave them and my passport in the hotel room. I keep the money clip in my front pants pocket.
Hotel: Hotels in nice neighborhoods in Paris are expensive. I recommend avoiding the extra 10 EUR/night/person fee for breakfast. Every corner of every Parisian neighborhood has a bakery and a cafe next door. Enjoy these. Save money.
I have traveling extensively throughout Paris and Italy and can offer various recommendations. Paris is HUGE! Get familiar with the Metro system. You'll spend a few days alone just scratching the touristy stuff off your list. The Eiffel Tower is indeed tourist trap hell. The Musee d'Orsay is a better experience in my opinion than the Louvre. I also highly recommend the Marais district as well as the Montmartre area. But the oft-overlooked area of Belleville is also worth the visit and a way to leave the tourist hordes behind. But by far my personal favorite Parisian neighborhood is the Latin Quarter, especially around Rue Mouffetard.
I also made a day trip to Omaha Beach and the American Cemetery. It can be done in a day if you leave Paris early. I traveled from Paris to Bayeux and took a taxi from there. But if you want to do it right, I would head out there, rent a car, and consider staying over night in a B&B. There is a lot to see and the public transportation is pretty much nil. Either you rent a car or pay for taxis. They restrict the public transportation options on purpose so as it funnel tourist traffic into expensive tour groups.
As for Italy, I have spent time in Venice, Udine (where I have family), Bellagio on Lake Como, Milan, Modena, and Cinque Terre. I have not yet been to Florence or Rome.