Kauai is a wonderful island. It is quiet, and more laid back than Oahu, you can find more of that "Hawaii" feel when you stay in Kauai. The activities are endless here, they offer paddle boarding, snorkeling, zip lining, horseback riding, 4 wheel ATV driving, boogie boarding, surfing, helicopter rides, catamaran rides along the amazing Napali Coast, and so much more! Kauai is an island endless with fun activities for couples and families alike.
Poipu is a great place in Kauai! It is on the sunnier end of the island and has incredible beaches. If you are wanting to see Monk Seals and Sea Turtles I recommend staying in Poipu. At the Poipu beach you can see the Seal and Turtles everyday, you can even walk within about 5 feet of them without them running away from you! It was such a fun experience for our family to see these beautiful creatures in their natural habitat and so close up!
On the west side of the island is Waimea, its not a very lush area and the beaches aren't incredibly wonderful either. You are a short drive into the mountains of Kokee Park and Waimea Canyon, so unless you are going to either of those places, I would recommend not staying there.