Taiwan itinerary

By combining an early start with Taiwan's high speed rail system, you could easily visit Tainan as a day trip from Taipei. Lonely Planet's Taiwan guide outlines a walking course of central Tainan that takes in many of the historic temples and other sites. If a day trip seems too rushed, you could take the high speed train to Tainan and explore the central area on the first day, then the next day take a bus or taxi to the Anping district, and return to Taipei that evening via high speed rail.

Even staying a night in Tainan would still leave you with plenty of time to spend in Taipei and its surrounding areas.

If you're mostly interested in traditional architecture, I'd recommend a trip to Jinmen. It's more expensive and only really justified if you spend 3 or more days there, but it has the best collection of traditional villages in the ROC that I've seen. Some say Mazu is even better.
Otherwise Tainan is definitely worth visiting, though unless you're a real temple junkie only 1 day is necessary- 2 if you're sure you won't get tired of temples.
Lugang is also definitely worth a visit. There's less to see than in Tainan, but it's more varied, with a very long old street and a couple mansions in addition to temples. It's also easier to get around. It can also be seen in a day from Taipei via HSR.
I recommend you get the Rough Guide instead of the Lonely Planet. It introduces more sights in more detail, and they are all arranged in walks.

My view of most traditional architectures in Taiwan is they are restored improperly. The "tradtional" is only in the facade. They are mostly reinforced concrete.

Am I asking too much? Don't say I didn't warn you.

Hi, You should absolutely go to Tainan, it's far far more interesting that Taipei if you are interested in historical architecture (in Taipei almost everything was rebuilt in the end of 19th and beginning of the 20th century by Japanese and next by Chinese again after 1945. Tainan, the first city in Taiwan and the oldest capital of Taiwan until mid 19th century, has not been rebuilt,. You'll find there remnants of Dutch architecture from 17th century, southern Ming architecture, Qing architecture, British colonial architecture.etc. the oldest Confucius Temple in Taiwan, and a very lovely district of old streets and Ming loyalist temple (I have forgotten it's name) across the street to Confucius temple, on the right from the main entrance (with a passage between the houses as narrow as 3 feet, close to the very famous 2nd hand book store) - and many shops with art creatioins behind.
The Red Tower (Chih-kan-lou, Fort Provintia) and Anping (Fort Zeelandia) are a must, but then close to the second one you have the First Old Street (Laojie) with its famous market and wonderful restaurants. Dont forget to eat the Coffin sandwich in one of them.
Opposite to Fort Provintia you will find the Guangong Temple (Ssu-dian wumiao, temple of Martial Arts) and Tianhou gong (Mazu's Temple "Later Heaven"), old residence of the Ming royalists.
Near Anping, don't forget to visit the British colonial merchant houses, the Tree House and the Salt Production Area, and many more. Tainan is evidently the most interesting city in Taiwan, and it is withing only 2 hrs ride by the THSR (High speed rail) - staying there for 2 days with the wandering through the streets in the night is an unforgettable experience.
Tainan is overlooked by most tourist routes, because it is far from Taipei, the main airport, except by those coming from the south (Filippines, Macau, Hong Kong), so unlike in Taipei where you can only hera cenglish, Japanese (sometimes also Russian) as foreign languages, here you can hear Spanish, Portuguese and Dutch as well - and you will really feel the atmosphere of a historic city there. And the Tainan night markets have definitely different atmosphere than those of the north, go e.g. to Hua yuan ye shi (Flower Garden Night Market), it's not a permanent market, with shops, like to\hose in taipei, it's open only on two or three days of the week, so it is more like a Jahrmarkt in many European cities..
I am going to Taiwan again in the middle of May, this time with my wife, and we are planning Tainan as one of the most important points of our trip,

I hesitated to mention "Architecture, specially old" before,because as a Japanese person,it reminds me of those below.

Presidential Office Building, Taipei
http://en.wikipedia.org/wiki/Presidential_Office_Building,_Taipei

National Taiwan Museum
http://en.wikipedia.org/wiki/National_Taiwan_Museum

Bank of Taiwan
http://en.wikipedia.org/wiki/Bank_of_Taiwan

Taipei Postal Office
http://en.wikipedia.org/wiki/Taipei_Postal_Office

Kaohsiung Old Station Building.
pictures (in Japanese)
http://taiwaning.zening.info/takao/old_station.htm
description (in English)
http://eng.taiwan.net.tw/pda/m1.aspx?sNo=0002121&id=A12-00070

Sun Yat-sen Memorial House and Yixian Park, Taipei
http://mykafkeasquelife.blogspot.jp/2012/08/sun-yat-sen-memorial-house-and-yixian-park-taipei.html

Knowing these things beforehand can be a kind of preparation,on second thought.

As for Tainan,I skipped it during my trips,so couldn't mention it.

Thanks for all your input. It looks like I should include Tainan. Sorry for responding so late, I had computer problems.
One more question, should we buy train tickets to Hualien and Tainan in advance or they usually are available a day or two before?

I am doing Taiwan and Tainan sounds like a worthwhile detour.

I'm working on Taiwan trip itinerary for my solo trip in March 2013. Anyone would like a copy can send me a message =)