Dancing and drumming are THE national "sport" of the Cook Islands and a visit to an island night is a must do. And there are plenty to choose from. The Staircase Bar and Restaurant (Thursday and Friday) tends to have the best dance troupes but unfortunately the food doesn't match the outstanding performances. But it would still be my recommendation. The Rarotongan Beach Resort and Spa (Wednesday) lays on a great feast, but the show itself, although very entertaining, is bit too touristy for my liking.
The Pacific Resort (Monday and Friday), Edgewater Resort (Tuesday and Saturday), Crown Beach Resort (Tuesday), Highland Paradise (Wednesday), and Manuia Beach Hotel (Saturday) also have their own shows. Prices typically range from NZ$35 to NZ$80 (approx. UK£16-37, US$24-56,18-42 Euros) for the show and meal. Some will also let you just pay for the show, but you don't get the best seats.