You should spend at least an entire day in Volcano National Park.  There are many things to see and do.  Here I am at the Kilauea Crater.  They have a very informative visitor center here, with all kinds of history of this Volcano.  For 100 years, between 1820 and 1920, the crater you see behind me was an active lava pit with fiery lava spouting and spewing from it.