Money Exchange: The majority of places in Madrid accept major credit card providers, but it´s recommended to also take out some euros in cash (you can find ATM locations throughout the city) to have in case and especially if travelling outside of Madrid to a smaller city or town.
Phone Services: You can check with your U.S. carrier regarding extra charges for roaming in Europe. Verizon, for example, offers an international travel pass for $10/day, which you´ll need to activate first (https://www.verizon.com/plans/international/international-travel/travel-pass/).
If your carrier does not offer such a plan and you prefer, you can also purchase a pre-paid sim card once in Madrid. These are sold at the airport or in the stores of any European carrier in Madrid (Orange, Vodaphone, Movistar).