Hey, I went to Kona last summer. Of course, Hawaii Volcanoes National Park will be the highlight of the visit. You will be able to spend a whole day there. Despite what some of the locals say, the drive from Kona to the park is NOT as long as they say, that is, if you aren't afraid of passing slow-pokes.
Don't expect to see lava. And when they say it is a tough, long hike over the lava-rocks to the spot to see the lava go in the ocean clearly, BELIEVE THEM!. The walk is at least 4 miles long round-trip, and in the heat of the day, it is a pain in the A**. Also, when coming back at night, you will NOT make it back unless each person in your party has a flashlight, it is pitch-black when going over those rocks. Also, bring a wind-breaker/jacket. We went in August and it was pretty chilly up there.
What we did, was we drove around the entire island for our 3 day visit. We made it back with time to spare, it was definitely a nice experience. In Kona we stayed at the Outrigger. It's an excellent hotel right on the beach within walking distance to a great snorkeling spot. The hotel rooms are standard but the location is prime. Not the resort-like hotel at all though.
There are a few but pretty good beaches on the Kona side, you'll just have to spot those by eye like we did.
If you venture over to the other side of the island, don't expect anything great. Hilo is pretty boring and it's always rainy. Akaka falls is alright, but nothing spectacular.
Have fun, and remember what I said about the volcanoes national park!
The proper term is "on final" not "on finals" bud...