One of the most exciting things about visiting Volcanoes National Park is an opportunity to see molten lava. This is somewhat a matter of luck. Sometimes when you visit, there'll be easily accessible lava flowing; at other times, nothing at all. When we visited, we had the opportunity to see lava glowing within a deep pit in the Halema'uma Crater near the Jaggar Museum.
The best time of day to see the lava is around dusk or even after dark. The bright orange glow becomes more apparent the darker it gets. The Halema'uma Crater is dangerous because of the toxic air fumes released, so you aren't allowed to approach it -- just enjoy the views at a distance.