Your holiday sounds fantastic! There are SO many things to do and see in Krabi province and also in the surrounding area. The south is quite easily traveled around opening up the doors to many possibilities depending on your interests. For the more adventurous there are a large variety of tours and guides who will take you all over for hiking, kayaking, canoeing, sightseeing, island hopping and more. Ofcourse there are also many things you can do without a guide or tour as well.
A few key places off the top of my head is the cluster of 80+ islands around Ao Nang, Hong Island, canoeing snorkleing all around, rock climbing for beginners and up and a visit to Koh Phi Phi if you're interested. You can visit a large number of other islands in the area very easily with a combination of transportation including bus and water taxi. There's also Phra Nang Beach, Koh Lanta National Marine Park and the island itself, exploring the many area caves like the Viking Caves, Benja National Park, the fossil shell cemetary, the Tiger Cave Temple, beautiful Wat Tham Seua, Krabi Town markets, cooking classes, overnight camping trips to area islands and the list goes on!
Krabi is wonderful as a main destination and offers a much more laid back atmosphere than some of the more popular islands like Samui, plus you have access to many places you can get to pretty easily. Take a look at a southern Thai map when you have a chance and you'll be able to spot plenty of places all along both sides of the coastal peninsula. You'll find a large variety of accommodatin choices perfectly sited for families and tons of things to keep you occupied and happy!
This is one of y favorite countries in the world! I've spent five holidays around Krabi and haven't run out of things to do yet! Have Fun!!