Hey Fella,
yeah, I was there about 6 weeks ago now. I stayed (deliberately for kiteboarding) in Nai Yang, which is 20mins from airport. The entire beach is more or less dedicated for kiteboarding. And whilst it's tricky at high tide to launch, it's not that difficult either. It's pretty sheltered from the waves (apart from one spot which you can keep clear of).
See below wind guru link for wind prediction.
www.windguru.cz/208477Also, there are about 4/5 different companies that rent kites there, so if I were you, I'd just rock up, and rent the right size kite and board for the conditions. You can also switch the board if you want. Fancy trying a directional for the first time, sure. I used these guys, but I have no officiliation so can't comment if they were better/worse/cheaper/more expensive than anyone else:
www.kiteboardingasia.com/They have a lot of Cabs and Airrush and there's another Germanish dude who has Norths.
Whilst there are few obstacles on Nai Yang (couple of boats and mooring lines etc.) it's pretty clear open and clear.
Personally Krabi is fine for surfing, but even in off season, I personally wouldn't as it's always busy, always has many kids and tourists wondering up and down. If you're already staying in Krabi, it will take about 45-1hr to Nai Yang.
Also, I think these guys have somewhere on the south east coast if the and again they do rental:
kitesurfthailand.com/Finally finally, there is a pretty decent wakeboard cable park in Phuket which is about 20mins from Krabi. It fills the low wind days... and if your'e going with the missus, there is a (respectable) massage place on site.
PM if you need more info.