At some resorts, when you're finished sightseeing or swimming for the day, you can enjoy a traditional Tongan feast. You will feast on lobster, crabs, pork, chicken, taro and local fruits, and afterwards be entertained by local dancers, and there may also be a kava ceremony.
Even outside feasts, Tonga has a good nightlife – you should definitely have a beer at one of the local bars in Nuku'alofa during your stay but remember, the later the evening gets the more raucous it becomes and Tongans are renowned as good fighters.
The best place for tourist nightlife and dining out is in Neiafu Town on Vava'u Island. Neiafu's pretty waterfront is a buzz with activity and whale talk during the high season from June to September and there are a dozen excellent restaurants to enjoy the locally caught fish, crabs and lobsters as well as a couple of lively bars. However, visit outside of these months and you'll find few tourists and some restaurants closed during the low season.
Despite being the capital town, nightlife in Nuku'alofa is very tame and there are not a great many restaurants to choose from. If you like Italian, head to the Waterfront Lodge and there's always great fish on the menu at the aptly named Billfish Restaurant which is also a minute walk along the waterfront opposite Queen Salote Wharf. There are a couple of bars / nightclubs in downtown Nuku'alofa but don't expect too much action unless its Friday night when local Tongans come out in force.
If you're after local entertainment, traditional dance shows are performed regularly for tourists, often at a resort location and including a buffet style island feast featuring pig, fish and root crops baked in an underground earth oven.