Thanks Jess -glad you found the info helpful.
As far as islands go, I've been to Phuket, Koh Chang and Koh Samet. The first two, while having budget accommodation, mean either airfares or a LONG travel time by other means.

Samet is not really my cup of tea........ I think it's over-rated. It could be so nice, but in reality, most of it is filthy. From the arrival pier to the nightmare roads, there's litter everywhere and to my mind, . The 'roads' over the back are full of substandard huts.
This is appalling, considering that this is supposed to be a National Park and there's a 200 baht entry fee (think the corruption didn't leave when Taksin did!!!) Just my opinion - there will be many who disagree.

If you don't mind a 7 hour road trip from Bangkok, Koh Chang is good and has a big range of budget accomm. - some right on the beach. At night, lots of restaurants set up tables on the sand...very good. Good snorkelling and diving just offshore.
I'm sure others on here will suggest some of the other islands which might also suit you.