I can't help you there with diving, having only done it once in kenting a long time ago. I have heard that there's good/reputable Green Island tours that offer many options. You should check out their website. (If you tripadvisor it, there's a lot of reviews.)

It was either NYT or WSJ that wrote an article earlier this year hailing Taiwan as Asia's Portland. Now I'm not really sure if I agree, but I certainly can see some of their reasoning. It is ultra-safe, people are friendly and move at a slightly less frenetic pace than its neighbors.

Since I am also planning to do a 2 week trip there in December, I thought I'd share some of the memorable or interesting things I found out or plan to do in Taiwan (aside from the usual sight-seeing spots!):

1a) Stay in a Love Motel.
There are some BEAUTIFUL motels... more hotel than motel. Yes they lack concierge services, room service (?), gym facilities, but they are beautifully themed. They range from kitschy, cheesy Batman themed rooms to glamorous modern European themed rooms. A lot of these accommodations scream fun, and is a good departure from the normal experience you get in a hotel.
http://www.agoda.com/asia/taiwan/kao...ng_branch.html
http://www.agoda.com/asia/taiwan/tai...spa_motel.html

1b) Eat at a themed restaurant.
I love love love themed restaurants when done right. In Taiwan, people have a good attitude and can laugh at themselves I find. I love how crazy, and creative they are.

2) Go to Taichung (new area around the luxury stores).
Last time, I went furniture shopping in this area and realized that the newly developed area has become highly upmarket. (Think girl playing classical instruments in front of waterfalls in luxury apartment building lobbies.) I also want to visit their air gallery again. Huge exhibits. Last time I happened upon one of my favourite contemporary painter's most recent work.

3) Visit aboriginal villages.
I forgot where, but somewhere in the south, maybe around Tainan, there's a huge event every year in May/June where representatives of different aboriginal people come together for this festival. There's also a more accessible town outside of Taipei where you can experience some aboriginal culture(forgot the name of the specific tribe that lives in the surrounding area).

4) Okay last one! Stay at a Traditional Japanese style Onsen hotel.
http://en.rocketnews24.com/2013/01/2...els-in-taiwan-

I have always loved the traditional houses. These look beautiful! This time around I plan to stay at one of these and get some hot-spring time in.

I hope this helps other travelers. Would also appreciate input on what other unique things I can do (especially in winter!).