If you want to fly from Bangkok to a Thai island, Koh Samui or Phuket are your two options. You can catch cheap flights to either island from Bangkok. Koh Samui and Phuket are both touristy and developed, however Phuket is much larger and there are quieter parts of the island to check out. You can also catch a boat to Koh Phi Phi from Phuket which is a beautiful island and is where they filmed some scenes from the movie 'The Beach'.
If you plan to bus it from Bangkok there are other options such as Koh Samet which is a popular weekend island trip for Bangkok residents. You can take a bus to the coast and a ferry to the island. Koh Samet is supposed to be a bit more laid back but I haven't been there.
From Beijing, you would likely have to fly through either Hong Kong, Guangzhou or Shenzhen to get to Bangkok. I do know Hong Kong has a lot of flights to BKK. Emirates actually has a flight that is generally reasonably priced but you need to be able to get a cheap flight to HK to catch it.