No visa is required for stays less than 90 days for business and tourism purposes per the State Department website, however your passport must be valid for 3 months past your return date from Bulgaria (6 months is highly reccomended), and you must have at least 1 blank visa page. Please cross reference the State Department Website for recent travel requirement changes and more detailed information, basic information from the State Department is copied below.

A US passport is required for US citizens who are not Bulgarian nationals. US citizens are authorized to stay in Bulgaria for a total of 90 days within a six-month period without a Bulgarian visa. This law is strictly enforced. You can file an application to extend your stay beyond the original 90 days for urgent or humanitarian reasons, but the application must be submitted to regional police authorities no later than five days prior to the end of the original 90-day period. Travelers who have been in the country for 90 days and then leave will not be able to re-enter Bulgaria before the six-month period expires. Travelers using official or diplomatic passports must secure visas prior to arrival. Please plan and apply for your visa early. Upon entering the country, Bulgarian immigration authorities request that all foreigners declare the purpose of their visit and provide their intended address.