The Marianas' top activity among Americans is scuba diving and snorkeling. In additional to the coral reefs you might expect, the waters around the islands were the scene of fierce fighting during World War II and there are many ship wrecks and even rusting tanks stuck on the seabed.
Many Asian particularly South Koreans visitors come to the CNMI for gambling especially on Tinian, and karaoke/hostess bars. Saipan has a thriving but illegal prostitution industry, most of the workers being from China or the Philippines.