Tongatapu is the principal island of the group of islands that compose The Kingdom of Tonga. It's the island that international visitors fly into and is small enough to see in just about a day with the help of a rental car. I'd probably recommend 2 days though if you'd like to spend some time relaxing at the various beautiful beaches. For me the highlights were the great isolated beaches that can be driven to, and the ubiquitous sea wall that surrounds the coast almost all around the island, creating blow holes, rock pools and tremendous crashing waves. My advice, if you only have a single day, is to try to make it Sunday or Wednesday. If Sunday, you get the unique experience of church with the King. If Wednesday, you can go to the Tongan feast at 'Oholei beach. Note that on Sunday Tonga is post-apocalyptically empty; you should definitely stock up on food and water in advance.
I was told that foreign visitors with a valid driving licence are permitted to drive for 24 hours (check this is true for your own country), and that if you wish to drive for a longer period, you need to get a Tongan licence from the police station in Nuku'Alofa for T25. I was renting on a Sunday, meaning most places where closed, but the manager of the NOA guest house was able to set me up with a car for T60 for 24 hours. I suspect it might be cheaper elsewhere if it wasn't a Sunday, but I didn't verify.
Driving and navigating around Tongatapu is very straightforward. Get a map from your accommodation which features the main attractions, and it is pretty straightforward to follow the simple roads (or Google maps works!). Just keep an eye on your speed, as the limits are quite low in Tonga. Also watch for pigs and dogs on the road (Tonga really is the place for pigs!). The roads are in quite good condition, but side roads to attractions off from the main roads could be bumpy enough for a 4WD.
I started the day with a visit to the Free Wesleyan Church. At 10am each Sunday mass is held for approximately 1hr30mins. If you are lucky the King will be in attendance and be sitting toward the front-left, so you will be sure to get a good look! Even if you are unlucky and do not see the King the choral singing is really beautiful, and it's interesting to observe the Tongans dressed in their Sunday best including a stiff, straw skirt wrapped around them. Obviously make sure you wear appropriate clothing.
Driving from Nuku'Alofa head west along Hihifa road you eventually will reach the tip of the island and the landing place of Abel Tasman in 1643. It's just a plaque, but the shallow, crystal clear waters behind are very pretty on a good weather day, and I guess it's historically interesting to visit. At any rate, it's near the Flying foxes at Kolovai, so if you plan to go there, you might as well visit the landing site. Flying foxes are giant fruit bats, and if you haven't seen them before it could be fun to go check out the strange black fruit hanging from the the trees near Kolovai
If you are hot and bothered from the car journey by now, Liku'alofa beach and its saltwater swimming pool will provide merciful respite. Lots of colourful fish seem to frequent the pool, and I also saw a little seasnake (but don't worry they're mostly harmless).
This is a huge boulder in a field basically :-). It's a pretty impressive boulder I have to say, and it is a mystery to me how it got there. To find it follow the road to the nearby beach but turn off right down a dirt track just before you reach the beach at Kala'au along the Liku road.
On a stormy day, you'll be in for a treat here. The rocks and rockpools are very unusually shaped and awesome fountains of water are violently pushed through gaps in them to create a spectacular display.
The beach with the sea wall, rock pools and blow holes is really great here. I met some locals on the beach who shared with me some roast pork and Tapioca. The friendly isles indeed.
The cave was shut here on Sundays, but the beach is beautiful and the resort hosts a popular Tongan feast of high repute each Wednesday evening for T40.
Entrance is T10, you will led the short distance into the cave with a guide. The cave has awesome stalactites and a fresh water pool for swimming. Lots of locals, and indeed my guide himself, enjoyed making death defying jumps from upon high into the pool. I was content with capturing the action of film, but a swim would've been nice if I'd remembered to bring a towel.
The Trilithon (Ha’amonga ‘a Maui) is on the north-east of the island and the terraced tombs are near Lapaha (the sign had disappeared, so they take a little bit of looking for, or just ask a local. They were near the church cemetery). Neither impressed me that much to be honest, but you may like them if you are interested in Tongan history. The trilithon was said to be built about 1200AD by the king at the time as the entrance to the royal compound. The king of Tonga, Taufa’ahau Tupou IV, suggested in 1967 that the trilithon also had astronomical significance related to solstices and equinoxes, somewhat like Stonehenge.
Take a look at the Royal Palace in Nuku'Alofa, the official residence of the King. The grounds were home to a giant tortoise given to the then King by Captain Cook until its death in 1966.