There is no visa required for citizens of United States for a maximum stay of up to 90 days. The same is true for tourist or business visas.
Per the US Department of State’s Bureau of Consular Affairs, both Spain and Andorra do not require any visa. However, the Spanish Government is very strict with visitors who overstay the validity of their visa. Spain, as a part of the Schengen Agreement, is one of the countries where you do not need to show your passport when traveling to the nearby countries. Immediate deportation is not uncommon, so you must leave promptly after the 90-day visa-free period.