We stayed at the Hilton Waikoloa the summer before last. I went with Red Sail for morning boat dives on two days while we were there, and I thought they were just fine. If I had to do it again, I would totally stay put and dive with them again rather than drive all the way back down the coast to Kona -- with the one exception being for a night dive with the mantas. With Red Sail you just meet down in the lobby in the morning, and they take you on a van which goes about a quarter mile down the road to where their boat is moored, about ten yards off the beach. You have to wade into the water to get on the boat. It's just so easy. There are plenty of fabulous things to see within a very short boat ride. We saw octopus, frogfish, turtles, colorful eels, and all sorts of damselfish. There were swim-throughs and caves in the lava formations, and all sorts of interesting coral. You won't run out of places to dive and things to see diving Red Sail on the Kohala coast.
Your wife will probably be fine hanging at the hotel while you're diving. It's huge. They have a spa, water aerobics classes, morning power walks, tennis courts, endless halls filled with Asian art, time-share presentations, and covered lounge chairs by the private lagoon. (Speaking of the lagoon, you need to check out the snorkeling in the lagoon too. The viz is pretty lame - maybe 10-15 feet, but all the cool creatures you see diving seem to find their way into the lagoon too). Sign her up well in advance for the dolphin interaction program.
We rarely ate at the hotel - the food is overpriced. If you get a hotdog by the pool, expect to pay something like $12 or $14. We went to the Costco in Kona when we arrived and stocked up on stuff for breakfast, snacks, and beer. Roys is great and nearby for dinner. We enjoyed Bubba Gumps in Kona, and some of the other restaurants in the shopping center where Roy's is located. There was a place in Kawaihae we liked. Get "The Big Island Revealed" for other recommendations.
You absolutely must go to Volcano National Park, the highlight of our trip. I would plan on two nights there: drive there from Waikoloa one day via the far North shore of the island. Stop for lunch in Hawi. Take the inland road from there along the ridge (great views) and back down to Waimea, then head down to the east side and check out Hilo. It will be dinner time by the time you get to the town of Volcano. Eat at the Thai restaurant. Best food, and cheapest, that we had there. Next day, do the Kilauea Iki Hike in the morning. You go right through the center of a volcano caldera, with smoke still seeping from the cracks, then back up to the rain forest, and around the rim. That afternoon go down to the sea, where the lava buried the road. You have to park, and then walk along the road, and then scramble over the lava. Go when it's still light, but stay until after dark to see the glowing lava in the distance. The next day drive back to Kona around the bottom of the Island.
Have a great time!
