Don't miss Volcanoes National Park (be sure to take the chain of craters drive - and drive down to the ocean from the volcano. The "Place of Refuge" is closer to Kona (or ship docked first in Kona then cruised around to Hilo) than it is to Hilo, but we enjoyed a few minutes spent there. Akaka Falls is beautiful. South Point is nice.
You might want to visit the web site for each island (or GO.com for each island) and request information or print out directions, etc. from the net.
That's what we did to prepare for the trip.