San Marino is part of Italy.  Italy is a Schengen nation which means that US citizens may enter Italy for up to 90 days for tourist or business purposes without a visa. Your passport should be valid for at least three months beyond your intended length of stay, Swift reccomends 6 months.  You will need at least 2 blank visa pages, and may be asked to prove you have sufficient funds and a return airline ticket.  For recent travel requirement changes and more detailed information, please refer to the State Department website.