When you first fly into Kona you will notice a very stark, barren desert lava field. Don't get discouraged by your first impression. Beyond that is an island with 14 different climate zones and remarkable changes in scenery as you venture out. Once you see beaches like Hapuna, the impressive volcanoes, and the lush greenery on the windward side, you will soon forget about the view from the airport.
The Big Island of Hawaii has a much larger land mass than the other Hawaiian Islands. This is a big area to explore and you never see it all, since alot of it is impossible to access. Also, much of the island is privately owned ranchland, namely the Parker Ranch.
I have been to the island on 4 occasions beginning in 2002. Every time I go back I notice an every increasing amount of traffic on the perimeter highway. This is probably the only negative when visiting the Big Island. Just be aware that you could spend over an hour on a 10-mile trip back to the airport in Kona.
My favorite hiking areas are the trails on the northern coast. Do not miss the Waipio Valley or Pololu Valley viewpoint. The lava flows at Volcanoes National Park are impressive, but you probably won't see lava spewing from the cone unless you take a helicopter flight overhead during an eruption. What you may find are areas of lava flow that you can drive up too. Keep your distance and don't walk right up to active flows. There are unstable areas where people have broke through and been killed or severely burned. No photo is worth the consequence of that!