Diving Bikini Atoll. Bikini is a UNESCO World Heritage site and one of the most extraordinary dive destinations on earth. The atoll's lagoon holds a graveyard of warships — the USS Saratoga, the Nagato, the Arkansas — sunk during the 1946 Operation Crossroads nuclear tests. Coral has colonized them over the decades, and the visibility in the lagoon is stunning. Getting here requires a live-aboard boat and serious planning, but serious divers consider it a once-in-a-lifetime experience. The nuclear history is impossible to separate from the diving, and you should not try to — the ships sit where they were deliberately sunk as test targets, and the people who lived on this atoll have never been able to come home.
The lagoon at Arno Atoll. Just southeast of Majuro, Arno is one of the easiest outer atolls to reach and one of the most beautiful. Its lagoon — enormous, shallow in places, clear — is ideal for snorkeling, kayaking, and simply being on the water. The pace of life here is slower and more traditional than in the capital. Day trips from Majuro are possible; staying overnight on the atoll gives a much better sense of the place.
Kwajalein's wreck dives. Kwajalein's lagoon holds the remains of over 100 Japanese aircraft and numerous ships from the Battle of Kwajalein in early 1944. It is a different experience from Bikini — closer, more accessible, less dramatic above water — but underwater it is exceptional. Access is through Ebeye, the Marshallese community adjacent to the U.S. military base.
Majuro's Alele Museum. Small but genuinely interesting, the Alele Museum in Majuro holds traditional navigational stick charts (rebbelib and mattang), canoe models, and exhibits on Marshallese history including the nuclear testing era. The navigational charts — made from curved sticks and shells to map ocean swells — represent one of the most sophisticated non-written navigation systems ever developed.
The outer island experience. The Marshalls have 29 inhabited atolls, and most receive very few visitors. Taking a field trip ship to a remote atoll — Jaluit, Wotje, or Maloelap — takes patience and flexibility, but the reward is a Pacific that feels genuinely untouched. WWII ruins dot many of these atolls; Japanese fortifications still stand at Jaluit.