European Travel Essentials:

https://sierrainthecity.com/packing-list-for-europe/

Money:

Payment by credit card is always best option. Make sure to notify your credit card company that you will be traveling outside of the country!

In general, you do not need much cash, but if you would like to bring some. you can exchange USD for EUR:

1. At your local bank. Ask for their conversion rates.

2. In Madrid at currency exchange houses in the center. NEVER convert money at the airport!