Hidden Gems of Tonga

Hi
Im going to Tonga in a few weeks and dont really know much about the place, is it best to get off the main land as it is in most other pacific islands or stay on Tongatapu?
Are there any hidden Gems that are a "must not miss" that I may not know about?
Also is it possible to travel by bicycle/scooter around any of the islands?
We are on a tight budget so any tips or advice would be greatly apperciated!

Thanks

The best "hidden gem" I found in Tonga was the amazing island of Niuafo'ou - good luck. ;-)
Some info on it is in the Niuas link in my signo line.

But Tonga is one of those Pacific nations where there's actually a lot to see and do on the main island, even if it isn't the best one for beaches or scenery.