Went there last summer, and had a great time.
Sometimes on the shopping channels you have great deals, so look out for them. My package included flight from Taipei to Taidong, then boat to Green Island (all return), one night-two days accomodation at a lodge, with scooter hire; then another night at the Hoya Spa Resort in Zhiben Forest Recreation Area, all for NT$6000. Just the flights alone costs more than half that amount!
Green Island an intersting mix of natural beauty of mountain and ocean, with history and adventure! Scooter is the best and most convenient way to travel around. Don't worry if you, like me, don't have a license...police don't seem bothered about that, as long as you wear a helmet at all times.
Try the seaweed ice served in a big seashell! Go on a night safari and you may spot a few wild Formosan deers and shooting stars. And snorkelling is definitely a must, as is bathing in the 'Sunrise Hot spring' (Zhaori ), especially in the middle of the night.
Really should go there if you have the chance. It's probably one of the few places in Taiwan when you don't meet bus loads of tourists (though you do get the occassional 'scooter-gangs'). Try going on a weekday when there are not so many people around, and as I gather accomodation seems cheaper too.
Though avoid typhoon season, or you could be stuck there for days (which may not seem too bad, as every moment there seems wonderful).
There are some pictures and writings about my trip on my blog, so feel free to take a look.
http://formosa1984.blogspot.com/2005_06 ... chive.html
East Coast National Scenic Area (東部海岸國家風景區)
Green Island (綠島)
http://www.eastcoast-nsa.gov.tw/en/trav ... p?travel=5