With all that water, watersports are the most obvious activity. The swimming's great, especially off Rarotonga and Aitutaki. Both of these islands also have excellent snorkeling opportunities, with high visibility and varied marine life.
There are several great places for hiking. Rarotonga's Cross-Island Track and Atiu's Vai Momoiri Track are two of the best hikes, both great views and neither taking more than a few hours. Cycling is popular on Rarotonga and Aitutaki, where you can rent bikes and cover more ground. Cavers have lots of choices, especially on Atiu, Mauke, Mintier and Mangaia. There's also good cave diving on Mauke, though you'll need to be certified and ask around for the caves' locations.
Though the missionaries managed to stamp out the practice on most of the other Cook Islands, tumunu, or bush beer-drinking 'schools', are still held on the island of Atiu. Descended from the Polynesian habit of kava drinking, the tumunu managed to survive in the bush, where islanders once brewed oranges. These days they use hops, creating a flat, warm, strong brew quaffed in an elaborate ceremony involving music, prayer and short introductions from each participant. Visitors frequently participate in the tumunu.
The Cook Islands are famous for their dancing, and visitors are encouraged to get up and get down. Friday night is the big night for putting the hips in motion, especially on Rarotonga and Aitutaki, though there are 'island nights' on most of the inhabited islands.
