Although Tonga's preeminent hiking areas are on 'Eua and its only golf course is on Tongatapu, it is Vava'u that has the most to offer water sports enthusiasts.
Vava'u is a famous sailing locale with one of the South Pacific's largest yacht charter operations. It's also perfect for ocean kayaking with lots of lovely protected waterways; a kayak touring company operates in this area. Deep-sea anglers too will find Tonga's top charter fishing boats based here.
There's also undeveloped potential for windsurfing at Vava'u, but the mecca for regular reef-break surfers is Ha'atafu Beach on Tongatapu. In Samoa the surfing waves tend to be far offshore (boat required), while in Tonga you can often swim out from shore. It's no place for beginners, however. Southern swells arrive May-September, northern swells from December-February.
Scuba divers are well catered to by professional dive shops in Nuku'alofa, Lifuka, and Neiafu, with many outstanding diving possibilities.
Snorkelers have even more options, beginning with the island resorts off Nuku'alofa, all of which operate day trips by boat. The finest snorkeling off Tongatapu itself is reputed to be at Ha'atafu Beach. At Ha'apai there's excellent snorkeling at the Captain Cook Beach Resort on Uoleva Island. At Vava'u, visitors can get in some excellent snorkeling by taking a day excursion from Neiafu by boat.