One thing you can try to book if you're going to Volcanoes National Park is a guided hiking tour of the Wild Lava Tube or Pu'u Po'o. The trail to this hike is only known to the park rangers and not available on any map. They only do this hike with one group once a week - on Wednesdays; and they take a maximum of 12 hikers. You have to be in decent shape to do the hike; but not a tri-athlete.
To reserve a spot, call the Volcano National Park number (808-985-6017) the Wednesday prior to the day you want to do the hike (exactly one week in advance). The park offices open at 7:45AM Hawaiian time. I suggest you try call them at precisely 7:45am, because the hike is usually booked within 20-30 minutes. You can make reservations for a maximum of 4 people. Best of all, the hike is free!
Because my wife had minor surgery 2 weeks prior to our trip last May, we weren't able to do the hike ourselves. However, I booked it for a couple of friends of mine who vacationed with us, and they said it was awesome.:cool3: