Went there for christmas. Stayed in Poipu and would go back there again. It was a great location, places to eat, walk to spouting horn, drive to waimea, jogging by the ocean, beach, snorkeling... Some pricy resorts, but probably you can find some cheaper places that are near the water.
We also talked about staying in Hanalei if we go back. it's on the north end, much more laid back, and probably cheaper. One of the most beautiful beaches in the world.
Ditto the catamaran ride to Na Pali. Amazing! If the waves are up, it can be quite wet. We did Captain Andy's, but there are several good companies. Good food, good drinks, good service. Saw more whales and dolphins than we could count.
Personally, I had a nice time renting a heritage and riding around for the day. There is basically one highway, which goes 3/4 way around the island, so wherever you go, you have to come back the same way. Traffic was bad near the city, from about the dealership, through town, and 1/2 way up the east side of the island.