Volcanoes National Park is a must see!! Here's what to do:
If you're staying on the Island of Hawaii, it's easy. If not, it's worth the side trip. Inter Island flights are very frequent, almost always available and relatively inexpensive. Take an early morning flight to Kona or Hilo. Hilo is closer to the Park but, in my opinion, Kona is nicer. Make a reservation at one of the hotels. I happen to like the the Mauna Lani Bay Hotel in Kona. Rent a car at the airport as soon as you land and drive to the Park. Have lunch there...the view of the crater is super as you see the steam rising. Then take the rest of the afternoon to drive around and see the sights. Definitely drive down the Chain of Craters Road. Be sure to get out and walk around and see the lava tube.
In the evening, have a snack and go out to see the lava flow after dark. It's wonderful, even from a distance...looks like a flowing orange river. Ask a ranger where to go for the best view as things change almost daily. You need good waking shoes and a flashlight for each person. Be sure to look up at the stars...spectacular!
Drive back to the hotel that night and rest over. Stay of a few days, or take the next flight back in the morning.
It sounds like a lot of hassle. But believe me, it's worth it.