Why Taiwan - that's a bit of a detour isn't? Thailand and Malaysia are obviously bordering countries, but that must be about a 4 hour flight over to Taiwan?
My main tip would be not to try and cram too much in, although admittedly it's one I rarely heed myself. A 21 day trip will become more like a 14 day holiday fitting that much in as you basically write off a day, sometimes more - especially getting there - every time you move places. I'd suggest hitting the beach straight away. Bangkok is a great city (or at least use to be), but it's not nearly as alluring when you're jet-lagged and adjusting to the heat. Far better to do that on the beach and if you're planning on spending time in Bangkok doing that on the way back.
If you do go to Bangkok, and want a
good hotel I'd recommend the
Peninsula. I regret not taking the private helicopter transfer from the airport, when I stayed there. You might need to shop around for some decent rates, as I'd imagine their headline rates are a bit pricey and you might get quoted some astronomical figures from UK travel agents.
Vertigo bar at the Banyan Tree is a must for cocktails at sun-set for the view, and
Bed Supper Club was the best nightspot last time I was there (a fair few years ago, admittedly).
What coast you should go to, will depend much on what time of the year you go. I went to Ko Phangan in Thailand and Tioman off Malaysia. If a small, basic, beach hut on the sand, in amongst the palm trees, with the warm water lapping quietly at your feet is your type of thing, then both can do a passable impersonation of paradise.
I must admit, I haven't been back to Tioman, where I learnt to dive, for fear of it being developed and my picture postcard images of it being distorted forever. If you're a diver and 30m vis, immaculate coral reefs and water so warm that you feel as if you are in the bath is your type of thing, then the diving wasn't too shabby either.
Ko Phangan also boasts the Full Moon parties and you can't beat dancing away on the beach to some killer tunes as the sun slowly rises over the horizon (except maybe if you tell them you played MK's wedding they let you, as a world famous DJ, onto the decks to play through sun-rise).
Personally, unless you are going the luxury (ie Peninsula) route, I'd favour just rocking up at these places. I can't remember these places ever being full and if you don't like the look of it, or prefer another one more, you can just move on, but can understand if for your honeymoon you want it all sorted.
