From Krabi, there are a number of good sites you can take day trips from. My favorite is Koh Ha, which means 5 islands even though there are 6. Its a very easy dive site, beautiful lagoons, but unlikely to see anything big. Around PhiPhi island, The Bida's are nice (there's Koh Bida nai and nok or Bida inside and outside, and Hin Bida for bida rock). These are also easy diving. For me, the biggest risk is the boat traffic, so bring dSMBs.
It maybe a stretch from Krabi, but the best diving in that area would be Hin Daeng and Hin Muang (Red and Purple rock) which are pinnacles off the southern tip of Koh Lanta. Currents can get strong with all the cuts, and down current possible. But when I was last there, the soft coral was spectacular and whale sharks and manta's have been known to stop by.