If you plan in advance thru the rental company, getting a car at nearby Hilo airport is pretty easy. Alamo/National, for example, will pick you up at the pier in a courtesy van, and return you after (just be VERY clear on the time of the last van back to the pier).
The drive to Volcano park is easy and relatively speedy on well-maintained state roads - appx 30 miles I think. A year ago, Hilo also had the lowest gas prices we encountered in the islands (but still higher than most mainland locations). The benefit of this plan is you can experience the park to your personal level of interest and exertion. For example, we picked up picnic-style finger food en rotue, then hiked the Kilauea Iki trail: aorund, down into, across, and back up a lava crater from the 1952 eruption. It's moderately strenuous, so no organized tour is going to so this. This is just one example.
Keep in mind that the lava floes are often miles from the main roads in the park, over difficult terrain. Nonetheless, there is a lot to see, smell (sulfur vents) and feel (steam vents) at this unique place.
Lastly, by all means (!!) follow safety warnings. This is not a theme park! Putting oneself in dangerous circumstances could have serious consequences.
