Some good recommendations on Kauai have been put forth here by many....
I'll add this...for a great hike and beautiful views (weather permitting!?!) as Tevake suggested, try the hike at Ke'e beach far up on the North shore that takes you to views of the Napali coast. It's fairly intense so bring lot's of water. BUT make sure to start early as it gets very busy on that trail AND parking can be a real pain as there is very limited parking there.
If you're into deep sea fishing, I would highly recommend this guy:
Kauai Fishing Charters - Hawaiian Style Fishing located in Kapaa, Hawaii
A one man show on a fairly small boat but this guy knows where to hook 'em! Plus one of few charters that depart from Kapa'a instead of Port Allen.
Aloha and Mahalo!