AmEx? Leave home without it. (Does your friend work for them?) And any other pre-loaded card for that matter. The fees you will pay are excessive and there is no need for it. AmEx is the least accepted card in Europe. Visa and MasterCard are your best options.
Most credit cards work fine in Europe, although you can run into situations where one card might not work at a specific place so it is best to have at least 2 cards from different banks, like one Visa and one MasterCard just in case. Same for debit cards.
Get a no foreign fee credit card if you don't already have one and you have time to do so. The fees add up and can be as much as 5% per transaction on some cards. There are many options out there. My preference is the Capital One card that pays cash back. It works everywhere in Europe and you pay zero for using it: no annual fees, no foreign exchange fees, no usage fees.
Also, check to see how much your bank's ATM card charges for foreign use or that it will even work (some smaller banks and credit unions do not allow debit card use outside the US). Charles Schwab and Capital One 360 accounts both have zero foreign fee debit cards so you can open an account with either of them just for travel placing money into them as needed. If you have a no fee debit card, you can then pull out cash from ATMs as needed and not carry more than €100 on you. Most ATMs in Europe still do not charge a fee for use. If you run into one that does, just cancel and try a different ATM from a different bank and it probably will not charge you any fee. And make sure you only use bank ATMs. These will either be inside a bank lobby or on the wall right outside a bank. The ones you find in shops, bars, grocery stores, etc. are more likely to charge fees.
Since this will be your first trip to Europe, you may want to purchase €100 or €200 before you leave. If you bank with a larger bank, they can usually get foreign currency for you (give them up to a week to find what you need). This will cost a bit more than getting the cash from an ATM once you arrive Europe, but might give you a bit less stress while you get settled in. I never take euros with me since there are so many ATMs around and it is simple to get what you need on arrival.
Another important thing is many merchants and some ATMs now try to get you to let them charge you in your home currency (called Dynamic Currency Conversion or DCC). This is bad. Refuse it. When they do that, they are free to use whatever rate they want for the exchange and it is not in your favor and can be up to 18% over the official exchange rate. You will still be charged in your home currency on your statement since the Visa and MasterCard networks convert the transaction into your home currency for you anyway before your bank even see the charge, but at the best possible rate.
And of course always inform your card issuers when and where you will be traveling outside the US.