First post in a while on here as I've been busy with work and family visits.
I've now been to Green Island twice; both times with family. The island is beautiful and very easy to get around. One road rings the coastal regions, and it is from this the most interesting sites are found. Everyone knows the former prisons and the re-education camp from the White Terror period, and they are well worth visiting. The Little Great Wall, Guanyin Cave, and the seaside hot springs are also great, but firmly on the tourist trail too. I wanted to put a few less visited sites here, some of which I only found on my second visit. All of them are on the coastal road and I will put them in clockwise order from the main town of Nanliao. Inland there is the Sika Deer Reserve, which I found a waste of time on both visits, and the Cross Island Old Trail, which was also fairly boring.
1. Prison Area Ruins
Green Island has a few interesting prisons and outbuildings that have not been redeveloped as museums. Within the New Life Camp compound there is a line of buildings that have been left untouched since the prisons abandonment. Most seem to be former offices and support buildings, and they still have a fair few artifacts within. The bleakness of the prison hit especially hard here (and in the well visited Bakua Building); the only sounds being the wind and the distant waves.
Nearby there is another prison from the 1980's that served as a vocational training center. The main buildings (behind a still in use office) are well closed off and surrounded by high prison walls. You can view the whole complex from the road as it climbs the hill. I've found no online evidence of people exploring here, and couldn't see a way in myself. On the road before you reach the old prisons, just past the new prison, there is a large, decaying hotel building that looks interesting. I didn't enter as my parents were with me, and they don't share the curiosity I have for these places.
2. Swallow's Cave and the Prison Cemetery
Follow a dirt track beside the white prison. The cave is signposted. You will come to a small bay hemmed in by large headland cliffs. Sitting in a ragged jumble on the slopes is the prison cemetery; the '13th squadron' of a camp divided into 12 prisoner squadrons. The prisoners in Green Island were all incarcerated for political reasons, which makes this spot all the more tragic.
Across the other side of the bay is the large Swallow's Cave. This is another eerie spot, despite the impressive hollow and the carpet of wildflowers which leads up to it. Here prisoners were cremated, and by some accounts executions took place within. There is also a stage in one corner which the prisoners built. I was surprised this cave had so many grim uses alongside more light-hearted ones. The cave held charred animal bones on my visit, and an air heavy with desolation, so I didn't stay long. I would still recommend going to these sites. It's important what happened here isn't glossed over or forgotten, and seeing them in person leaves a real imprint.
3. Niutou Hill
I get the feeling most people zoom past this spot on their way to the well known Guanyin Cave and Little Great Wall. It's the remains of one of Green Island's ancient volcanoes, and it only really reveals itself after making the climb up past the pillbox. It's an impressive headland with peaks of volcanic rock jutting out of a lush meadow (these are supposed to resemble cow horns; niutou translates as cow head). Beyond the cliffs a series of rocky islets and stacks stand among the waves.The top of the hill gives a good view over the prison complex and numerous other dramatic cliffs and rocks. The entrance is found at the top of the steep hill coming out of the prison.
Sign-posted, and one of the better known sites here. But still I think a lot of people miss it, as it's down a very steep hill to the coast. The unusual aspect of this bay is the abandoned village found just back from the beach. There are a number of basic rock and coral dwellings, most of which are roofless, but in this wild setting they are enigmatic. The bay itself is also beautiful, and a large hollow has been eroded into the cliffs on one side.
After Youzihu some of Green Island's more famous spots are found, along with a long downhill back to the coast path. I'll mention Fanchuanbi head, as like Niutou Hill it's much larger and wilder than it looks from the coast road. It also juts far out to sea, becoming more dramatic the further along you walk. A lot of goats seem to live up here too. The hill is easy to reach via a wooden staircase above the seaside hot springs facility.
6. Green Island's Reefs
Of course a lot of people know Green Island is excellent for diving and snorkeling. Most non-divers however get tied to a group being pulled around on a completely lame 'snorkeling tour'. If you are a reasonably good swimmer, as there are some slight currents, it's rewarding to snorkel independently. I found two great spots are at Dabaisha, where a blue and black ringed seasnake swam calmly beneath me, and at the beach just south of Nanliao Harbor, where there is an enormous mushroom coral that is around 1200 years old.
Thanks to many Taiwanese having one or all of the following; a distrust of the sea, inability to swim, dislike of the sun (the beaches are quite exposed with no umbrellas), or a quick photo-stop group tour mentality, Green Islands beaches are often deserted, even on weekends. This has led to an abundance of protected and largely undisturbed marine life. The water can be tricky to enter sometimes, especially at low tide, as a shallow platform of uneven rock and coral extends past the helpful wooden piers. Perhaps reef shoes are a good idea. In any case if my 79 year old grandmother can manage it so can most able bodied people. The coral and fish species are vibrant and diverse and I have spent hours swimming around them.
More details about other sites on Green Island can be found on the Hidden Taiwan Map here. There are regular ferries from Fugang Harbor near Taidong year round. They are notorious for seasickness. Sometimes there are very early morning departures, so it's best to stay a night in Taidong before leaving. Try and visit the island on a weekday, as it's packed with obnoxious petrol scooter tours on the weekends (despite available e-scooters and e-bicycles). You may get lucky and arrive during a bit of a party (e.g the Mazu Festival parade, complete with guys hitting themselves with sharp knives)!