In reply to Jimrich88:
I recently went from Phuket to Krabi. We took the ferry from Phuket town which cost 1000 baht each (about £20). On the way, we stopped at Ko Phi Phi for a day (where there is some climbing as well), before carrying on. The ferry takes about 4-6 hours so is a bit slow, but the stopover made it better.
Another option would be getting a coach from Phuket to Krabi. It takes about 3 hours and is cheaper than the ferry or a taxi, though I can't quite remember how much (we did this on the way back).
Once you get to Krabi then grab a taxi to Ao Nang and get a longboat to Ton Sai. If you want to go to Railey, you can get a longboat from Ao Nam (careful as the two sound similar). Railey and Tonsai are within easy walking distance of eachother (best route is over the beach at low tide).
Koh Yao Noi sounds like a good suggestion but we didn't go there.