Aitutaki Lagoon is the single most spectacular sight in the Cook Islands and ranks among the great lagoons of the Pacific. Fifteen small motus (islets) ring a vast expanse of turquoise water in every shade from pale aquamarine to deep green. A lagoon cruise — typically a full day on a boat with snorkelling stops, a BBQ lunch on a sandbank, and a visit to One Foot Island where you can get a passport stamp — is the default activity and deservedly popular. Even if you are based on Rarotonga, a day trip to Aitutaki by Air Rarotonga is worth doing.
The Cross-Island Track on Rarotonga is a challenging but rewarding hike through the volcanic interior, culminating near Te Rua Manga (the Needle), a dramatic basalt pinnacle rising from the jungle. The full walk takes 3–4 hours and requires reasonable fitness and decent footwear. The contrast between the coastal tourist strip and the wild, dripping interior is striking.
The Muri Night Market on Rarotonga, held Thursday to Sunday evenings, is the best place to eat a cheap meal, drink a cold beer, and watch local life. Stalls sell grilled fish, ika mata, fresh coconut, BBQ plates, and local sweets. It is crowded, relaxed, and lit by fairy lights on the edge of Muri Lagoon.
Whale watching between July and October can be done from the Rarotonga foreshore with binoculars, or on dedicated tours. Humpback whales pass through on their annual migration and sightings are fairly reliable in August and September.
Atiu is the most rewarding of the accessible outer islands: dramatic makatea (raised coral limestone) terrain, caves with swiftlets, birdwatching, and a strong local culture largely untouched by tourism. It requires more planning than Rarotonga or Aitutaki but delivers a very different experience of the Cook Islands.
Cultural dance shows on Rarotonga showcase traditional Polynesian drumming and dance — polished performances that nonetheless give a genuine sense of the islands' living culture. Most larger resorts host them weekly; the Te Vara Nui Village show is one of the better-known options.