I spent time on the Marianas/Caroline Islands as a grad student. Going to the bottom of the stairs is enough at Majuro, unless you have an urge for Spam. Truk (Chuuk) has fine snorkeling and diving around some Japanese WWII wreckage in shallow water - water clear enough that the sharks don't bother you. Pohnpei has excellent wall dives. The attraction is mostly snorkeling/diving. The Islands are used as a dumping place for out of date Japanese cars. Japan stimulates its economy by taxing older cars at high rates to encourage purchase of new ones. Old ones find their place in places like the Trust Territories or Far East Russia.