At the southern end of Krabi province is a string of large islands separated by narrow mangrove-lined channels. The southern-most of these is Lanta Yai, or "Big Lanta". While the island is somewhat popular, it is not nearly as developed as the other destinations around Krabi, and still retains some rural southern charm. Aside from the charms of the island itself, there are some fantastic diving opportunities around some of the smaller islets in the seas around Lanta.
Lanta is actually the closest jumping-off point for the South Andaman's best dive site, Hing Daeng and Hin Muang. You can easily reach the site in a day trip from Koh Lanta, although the best "avoid the crowds" option is to do an overnight live-aboard, which will also usually take in Koh Ha.
There are plenty of accommodations in all classes on Koh Lanta, from modest guesthouses to some truly world-class five star resorts. There are also some pleasant places to eat and drink, as well as some interesting land-side sights to fill your decompression time before flights. See the Thailand for Visitors guide to Koh Lanta for ideas.