Which islands or coastal spots should I visit as a first time visitor in Thailand?

Hello! I will visit Thailand for the first time next month, I'm flying to/from Bangkok, and also planning to visit the north, but I would like to visit some islands or coastal areas in the country too, and wondering what are "must see" or specially interesting spots for a first-time visitor. Ideally, some that are relatively easy to reach. I don't mind using plane, train, boat, or whatever is needed, I just can't make a super long journey on purpose because I'm in the country for just a bit more than a couple of weeks and wouldn't like to waste too much time on transport. Also, I know that some super touristy areas are still worth seeing, but I'd prefer to avoid the obvious and extreme tourist traps.

Any tips will be appreciated!