When you first fly into Kona you will notice a very stark, barren desert lava field. It really isn't that scenic when you compare it to the other islands. But once you see beaches like Hapuna, the impressive volcanoes, and the lush coastland on the windward side, you will soon forget about the first impression.
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 hwy. This is probably the only downside. You can spend over an hour on a 10-mile trip to the airport, so plan accordingly.
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.