Been to Kauai 2 times so no expert at all. Only giving what I know.
Staying at Poipu beach is really nice. The beach is not crowded. There is a mix of sand lovers, snorkelers and surfers.
The beach has stretches of sand ringed by rocky outcroppings on each side. This lets the surfers have a spot further out while the snorkelers and swimmers have calmer water. This pattern of sand, ringed by rocks is repeated several times, rather like individual pools with a barrier between. Its quite lovely.
I would not describe it as great snorkeling but it allowed me to introduce my daughter to snorkeling for the first time so that made it special. No coral, just rocks with various fish around.
Did not do any diving either time so have nothing to add in that regard.
The helicopter ride across the island is magnificent. Taking the helo trip without the doors was wonderful for photographs - no reflections. A 12-24 lens was nice to have in the copter. Take an SD/CF card with plenty of space on it as you might take many many photos and its nearly non-stop great views so no good time for a card change.
Hiking the canyon lookout trail at KoKee state park was wonderful. After the first hour, we never saw another person. It was like owning the island.