Thailand is an ideal destination for a romantic holiday. You can start with a three -day stay in the North, (Chiang Mai or Chiang Rai for example), and/or finish with a two day stay in Bangkok. Beside the amazing beaches, it is a great place for shopping, culture, nature and nightlife.
There are all kinds of beach resorts and accommodations in Thailand. Your choice will depend on your budget and how isolated you want to be from the rest of the world. Weather (and rain showers) is also an important variable: If you are traveling from May to September, stay in Koh Samui, Koh Phangan, or Koh Tao, from November to January choose Phuket, Krabi, Koh Lanta or Koh Phi Phi, in October choose Koh Samet or Hua Hin. Rest of the year is fine everywhere.
If possible, try to stay in two islands or two different locations.
You could stay a few nights in Koh Samui and then a few nights in Koh Phangan. Koh Samui is a large island with an international airport (and hospital). It’s a good start for your holiday as you will land there. Spend a few days, do some shopping, try different restaurants and then take the ferry to Koh Phagan or Koh Tao. Koh Phangan is famous for its wild parties (Full Moon,Half moon, Blackmoon, etc.) which is something you will want to run away from or run quickly to according to your age mostly. It’s also more authentic, wilder and less touristy than Koh Samui. The beaches are breathtaking: you will feel very close to paradise.
There are few luxurious hotels on Koh Phangan as it’s mostly for young backpackers, but here are a few recommendations: Santhiya http://www.santhiya.com/kohphangan and Rasananda (http://phangan-rasananda.anantara.com/)
Koh Tao is a small island reachable by ferry from Koh Samui. Less touristy than Koh Samui, wilder and more intimate, it is especially recommended for divers.
Another option if you want two different atmospheres: combine Krabi with Koh Lanta or with Koh Phi Phi. Krabi is not an island but you won’t notice the difference. There is an international airport. It’s wilder, and overall nicer than Koh Samui. There are three main beach areas Railay, Ao Nang, and Tubkaek. Railay beach is stunning. If you can afford it, stay at the Rayavadee (the only luxury hotel there). There are more luxurious options in Ao Nang and Tubkaek.
Take a ferry or speed boat to Koh Phiphi or Koh Lanta. Both are beautiful small islands. Check the Zeavola hotel and the Phi Phi Island Village Beach Resort on Koh Phiphi, and the Pimalai and Lanta resort on Koh Lanta.
You will need to take a ferry (or private speed boat) to reach the smaller islands, and therefore night transfers may not be available. Last ferry could be at 16:00. This is important if your flight lands in the afternoon or evening.
If you are staying in more than one hotel, always finish your holiday with a “wow”, with the most luxurious one.