Although Bangkok is not on the coast, you can still explore some incredible beaches nearby, in this guide from Kuoni Travel.
Chiang Mai & Chiang Rai
Hua Hin
Koh Yao Yai & Koh Yao Noi
Koh Lanta
Koh Phangan
Khao Lak holidays
Koh Samui
Krabi holidays
Phuket
Koh Phi Phi
Koh Samet
Koh Chang