There's lots to do in Krabi

Scuba and Snorkeling

Turtle at Krabi dive site
 

Krabi is a haven for scuba diving and snorkeling. There are several local sites offering good, adventurous caves and caverns, plus some fabulous dive locations: Shark Point, Anemone Reef (as the names say it all, great for leopard shark watching, covered with anemone in bright plethora of colors), and Phi Phi. You can take diving course and get a PADI certification while you're there.

Rock Climbing and Trekking

Take a day trekking tour to Emerald Pool. Definitely a trip on the land side is worth it; don't expect the wildest rainforest, but actually there is many places that come close. Emerald pool and Hotsprings are the most popular ones and also with most people. To avoid the crowd a little try to pay a few 100 of baht(2-4 euro) more and then make it a private tour. Then you can go new unexplored places or atleast not have to be depending on a whole bus of others.

Sea Kayaking

The huge mangroves and prehistoric limestone Karsts that are the hallmark of Krabi's outstanding geography, create a labyrinth of sea canals dotted with caves in an extremely rich eco-system that is only possible to explore by Kayak.

Many companies now offer guided tours to the stunning Phra Nang bay, especially at Boh Tor in the north of Krabi near Ao Luk, which is the main are for mangrove kayaking tours. Here you can paddle through sea caves with incredible stalactite and stalagmite formations, and also walk through a huge cave with many stone age cave paintings. There are even child-friendly companies that conduct 2-day courses.