Neiafu is Tonga's second "city," but it's still a sleepy little town of 6,000 inhabitants. It's a great place to explore on foot, visiting the markets or local attractions, shopping for handicrafts or groceries, dropping into a cafe or bar to hear yachties chatter, visiting travel offices to organize excursions, or hiking out into the unspoiled countryside all around. Neiafu is a much more colorful, attractive, appealing, and restful town than Nuku'alofa.
The longer you stay, the more you'll like it, and the better you'll become attuned to the relaxed pace of life. You get the impression that this is a place where everyone knows one another, and where things can be arranged on short notice. The only drawback is that no swimmable beaches exist near town.
Be aware that almost all shops, restaurants, and bars in Neiafu are closed on Sunday. The only people you're likely to meet on the street on Sunday are those coming from or going to church. Sunday nights a church service consisting mostly of hymns accompanied by rock music is broadcast across Neiafu on loudspeakers. This really is an unusual place.