On the seaside, the boat trip to islands like Koh Hong, Koh Pak Bia, and Koh Lao Lading is probably the most popular, and this is completly normal, these islands are beautiful! But there are variants like the tour that ends in style with the sunset at Railay Beach. You can also explore Phang Nga Bay from Krabi.
On the land side, the Emerald Pool attracts the most tourists. But some tours let you discover Krabi's countryside and local life away from tourists or the little-known Ao Luek bay area. Without forgetting a trip on Krabi river visiting Koh Klang island which I like very much!
Krabi can also be the starting point for a 3 days trip to Koh Yao Noi.