Have you already booked your air? If not, consider the Big Island for the volcano viewing. Go to the bar in Volcano House that serves Pele's Delight and has a straight on view of the hot lava. Drive around the island, a whole day trip, stop at the Punulu'u Bake Shop near the farthest point south in the US for goodies or lunch. Their malasada donuts are the best in all the Islands. See the volcano and big banyan tree in Hilo. Another lunch option is in Hilo where you can grab native fare and eat in the park near the ocean. Go to City of Refuge en route south out of Kona. Pu'uhonua O Honaunau is the Hawaiian name on the directional post.
Stay at Hilton Waikoloa Village for the gorgeous gardens and the personal Asian art collection of the developer. The spa is killer. Ride the tram or, better, the little boats. Take the little boats at night and enjoy the lights in the gardens. Eat at Roy's in the nearby shopping center. You can also rent a jeep and drive to the top of Mauna Kea to see the observatory, weather permitting. A beach is not on site but a lovely lagoon is. Beach is close, next to the Marriott. Ocean Tower has its own adult pool.
My husband and I find this a wonderful adult vacation and we return every few years.
Costco is near the airport en route to the hotel.