Hi
My personal suggestion regarding where to visit would be as follows:
Bangkok - single day just to acclimatise yourself, maybe leave a couple of days at the end of your holiday to do "touristy" stuff before leaving Thailand.
Chiang Mai - there's loads of different stuff to do up here, it really depends on what you want, whether it's culture, shopping, trekking, Thai massage,places of interest or just chilling, Chiang Mai has it all. You could easily spend 3 days here.
In terms of the islands & beaches, I would suggest:
South:
Phuket - largest island in Thailand & most commercialised, its located in the south of Thailand, so its a good base to island hop from, not a place personally that I would rush to though.
Phi Phi - small beautiful Island just south of Phuket. Very beautiful & a popular spot for all age groups. If you want to go and have a really great time, have lots of diving, bars etc...this could be a great spot. It's a little too busy for my liking, but many love it, also not as good value as it used to be.
Krabi - On the mainland near Phuket and Phi Phi. It's as beautiful as Phi Phi, but much more laid back and cheaper. It's famous for rock climbing
Koh Lanta - South of Krabi, very laid back quiet and relaxing. Not the most attractive island, but it's a favorite for anyone who just wants to kick back with a beer and a sunset. On my last trip it was far and away my favorite island.
Koh Samui - 3rd largest island is located on the east coast, probably the second most built up island after Phuket. Not to my liking, but if you want a good time, it does remain a very popular spot. Has some lovely beaches particularly Lipa Noi beach.
Koh Phagnan - North of Samui. This is THE young backpacker party spot. Average age on the island is probably early 20's and you can party on the beach any night of the week (and especially at full moons) at Hat Rinn beach. It's a big enough island that there is also many quieter beaches as well.
Koh Tao - This is the diving mecca for the young budget minded traveler. It's small, laid back, but with enough going on for a good time. And it's one of the cheapest diving spots in the world.
Close to Bangkok:
Hua Hin - very popular beach resort, the oldest beach resort in Thailand, the King has a palace here, masses of construction working always going on, always full of tourists, beaches ok, but sea can be a little choppy compared to the South. NOT a favourite of mine by any stretch of the imagination!, and quite pricey.
Cha Am - next to Hua Hin, much quieter, Thai holiday city, much better value, loads of great value seafoods available on the beach.
Pattaya - only been once, will never return, need I say more ?.
Koh Chang - 2nd largest island in Thailand, been "developed" heavily within the last 5-7 years, to my mind its been destroyed for the sake of commercialism !.
Koh Samet - island close to Koh Chang & Bangkok, not as developed as Koh Chang yet, a popular place for Bangkok weekenders, much more like Krabi than Phuket !!.
Suggestion - if you wanted to see as much as possible then I would suggest:
Phuket - Phi Phi - Krabi - Koh Lantra - allow 1 week, then onto Koh Samui - Koh Phangan - Koh Tao - allow another week, then Hua Hin - Cha Am - Pattaya (?) - Koh Samet & Koh Chang allow another week, so adding the time in Bangkok & Chiang Mai as well, its a month long visit just to scratch thew surface !!.
Regards
Kevin