Weather
Fiji is a tropical country with two seasons: wet (rainy) and dry. September is toward the end of the dry season, which continues until October. It's sometimes called Fiji's winter, and although nights are cooler and you might need a light sweater, most travelers will still consider the weather warm in September. Being the dry season there isn't much rain, but there's the chance of a shower.
Although temperatures are fairly uniform year-round in Fiji, expect high temperatures of up to 80°F (27°C) in September, which is a few degrees cooler than at the height of the wet season. Humidity is also much lower now, which many travelers find more comfortable. Meanwhile, the chance of cyclones is slight, and while unseasonal storms can't be ruled out, you're unlikely to encounter destructive, disruptive cyclones in September.
Crowds & Costs
The dry season generally is the peak season for travel to Fiji. However, September is quieter (and therefore cheaper) than July and August because many Australian, New Zealand, European, and North American travelers return to work and school after their winter/summer vacations. September is a busier time for travelers not constrained by school vacations. Book flights and accommodation in advance and expect to pay high prices, but there's likely to be more availability than in previous months.
Chat with a local specialist who can help organize your trip.
Where to Go
Many of Viti Levu's resorts are on the west and south coasts because this is the drier side of the island during the wet season. During the dry season, however, there's no need to avoid other parts of Viti Levu. Head to Nananu-i-Ra Island, a small island off the northeastern coast of Viti Levu. It's very laid back, with low-key accommodation options and quiet beaches. The windsurfing and kiteboarding are excellent here, and you can take lessons and rent gear.
Travelers interested in history and culture might also want to visit Fiji's only UNESCO World Heritage Site, Levuka on Ovalau Island: a historic town that was Fiji's capital until 1882 and once an important trading post and whaling town. Ovalau is well off the tourist circuit but can be reached by ferry or small plane from Suva.
What to Do
This month, make the most of the calm sea conditions by diving or snorkeling. Swim with manta rays at Barefoot Manta Island in the Yasawa Islands; check out some of the finest soft corals in the world in the Somosomo Strait between Vanua Levu and Taveuni, particularly at Rainbow Reef and the Great White Wall; or dive at one of the largest barrier reefs in the world at the Great Astrolabe Reef off Kadavu Island.
Conditions are also good for surfing in September, with consistent swells of between 8 and 10 feet (2.5 to 3 m.). The Mamanuca Islands are especially famous for their surfing, with some of the best breaks in Fiji, including Cloudbreak and Namotu Lefts. The waves here are mostly suited to intermediate and advanced surfers, but there are some beginner-friendly spots, and lessons are available.