I've recently done a 3 month stint around Thailand and further afield.
It completely depends on what you're wanting to get out of the holiday, as different areas of Thailand offer entirely different experiences.
I would strongly suggest splitting the holiday into two; one week for exploring, one for relaxation. Thailand has some fantastic beaches, and who doesn't love beaches (my girlfriend, she hates sand in her toes) but without actually getting out of the main tourist areas you could be laying on a beach next to dozens of Europeans anywhere in the world and save the 12 hour flight.
Koh Lanta is around 1hr 30 from Krabi Airport, and will cost around £15 in a shuttle. In Koh Lanta everything slows down. You will see the best example (within easy access) of an unspoilt Thailand. Stay in a resort around Long Beach, you'll see fire dancers nightly, giant crabs and the best sunsets in the world (Google them, it's incredible). Hire a scooter and go exploring. The roads are empty and it's super easy. You'll find empty beaches (usually to yourself), waterfalls, caves, etc. You can then go sea kayaking, four island boat trip, and snorkeling. The night markets are fantastic and not to be missed.
Koh Lanta was one of our highlights.
You can then easily take a boat from Koh Lanta to Phi Phi, about 2hr and again, a few quid.
The main port side of Phi Phi is a ghost town during the day (once you've broken off from the hoards of backpackers and got lost in the rat Warren of streets) with very little to do aside from the usual one or two boat trips (the plankton is fantastic, snorkeling cool too but stop offs very busy) hiked to the view point and you've about exhausted all the attractions.
With that said, you'll likely be nursing a hangover from a c. £3 bottle of rum & mixer bucket, until around 10pm when everything starts over. Great for a few nights, wouldn't recommend more than 3. However, if you take a long tail boat around to the other side of the island, you can find nice resorts on private beaches with literally zero amenities, except the beach and sea. This was too quiet for us though, felt slightly trapped.
You can then get a boat into Krabi and onto Ao Nang.
Avoid Krabi Town, nothing there. Tiger cave temple is cool, as is emerald pool but they're a distance out of the centre.
Ao Nang is nice, but after we'd been to Koh Lanta and various other islands, it was too busy for our liking. Not much to do other than beach, and drink. Nice for a week relaxing though. Not too dissimilar to any other beaches in Europe.
Not exactly the advise you were after but this is my experience, hope its useful.