and most of what I want to do is on Oahu, so staying at Aulani makes sense. However, the one thing I really want to do in Hawaii is see the Volcano National Park on the Big Island. Does anyone know if this can be done as a day trip from Aulani or should I plan on booking a couple of days at a hotel or resort on the Big Island?
TIA
As OP have said, it is possible but it makes for a very long day. I just returned from Hawaii where I spent a week each on the Big Island and Maui and a couple of days at Aulani. While I was on the Big Island, I did a Circle Island and Volcano Tour with Polynesian Adventure Tours(Grayline). We circled the entire island(288 miles or something like that) and stopped at the Kona Coffee Company, Black Sand Beach, Volcanoes National Park, Rainbow Falls, Big Island Candies and Macadamia Nut Factory. The tour was about 12 hours for those of us on the island, but some of the people on the tour were picked up and returned to the Kona airport. They came over from Honolulu and were the first people picked up and dropped off. I believe their return flight was at 7:30pm. Their day was very long, but it may be worth it to you to do the tour. Aulani lists some day tours on their website or you could search on your own. If you book your flight early enough, you can get a Hawaiian Airlines flight for as cheap as $79 each way. You can also fly into the Hilo Airport.
We only spent about two hours in the park on the tour. We went to the Jaggar Museum, steam vents, Devastation Trail and Thurston Lava Tube. There isn't much to see as you drive and we didn't do the Chain of Craters road(I think that's the name) down to the sea. If you're ok with not doing hikes and spending a limited amount of time in the park, the tour may be for you.
Your other option is to fly over on your own and drive to Volcanoes as OP have said. Keep in mind that VNP is about 2.5 hours from Kona if you drive south and a little shorter(I'm told) if you drive north through Waikoloa from the airport. Unfortunately, in order to see the Black Sand Beach you will either have to drive south and then return that way or drive around the entire island if you decide to go the Waikoloa way. If you haven't already done so, check out the Frommer's Big Island book which has an attraction map of the island.
I loved the tour and decided to take it because I didn't want to drive the 2.5 hours each way from Kona. I also wouldn't have known where to stop because there are no public bathroom stops along the way. The tour either stopped at a store or a site so that we could use the bathroom. The roads are similar to two lane country roads, so if you get behind someone slow you are out of luck.
Most of the attractions are on the Kona side of the island, but there are some on the Hilo side(Akaka Falls, Hawaii Botanical Gardens, etc.). You may want to look and see what you want to do in addition to the volcano to decide whether or not to stay a while. Good luck with your decision.