John, there are lots of travel guides and I'm sure members here will make suggestions... It's a pretty big island with lots to see. Flying (helicopter?) over the active lava fields will be a great experience. But a surface exploration of the Kilauea Caldera area and a drive from there down the Chain of Craters road to the ocean gives a very immediate feel for it. The road, as far as one can now go on it, disappears into the lava. Quite remarkable, and one can walk out over the lava flow eastward along the shoreline to get closer to the place where lava pours into the ocean.
Worth a drive too, down South Point Rd off the main highway to Ka Lae (South Point), the most southerly spot in the 50 US states. There's extensive blow-holes there, dramatic when the surf is up, and interesting to crawl around at calmer times.
From there around toward Kailua/Kona, there are Macademia nut groves and processing sites, interesting to visit. And closer to Kailua near the town of Captain Cook and on Kealakekua Bay there is an old Hawaiian sanctuary, kept now as a park. Good for a half-day visit. Famous Kona coffee is grown on the slopes above Kailua, still many small farms there.
To get to or from Pololu Valley, which is the end of the road at the north end of the island, I'd suggest driving Kohala Mountain Road, which runs along the upper slopes on the dry side below the ridge, and comes into Waimea (Kamuela), which is very near the Parker Ranch... another place to visit!
Around on the east (wet) side is Hilo that has a small-town feel and has less tourist development than the dry side. Flights over Kilauea area often originate here, with the sizable airport sharing inter-island duties with Kona airport. Up the Wailuku River from Hilo are a couple of big waterfalls worth a stop.
We're ready to go back any time!