Eat breakfast at the Volcano House restaurant one day. I could have spent the entire day drinking coffee and looking at the view.
Bring a small flashlight and see the crater at night from the observatory.
The Kilauea Iki hike was nice.
If you have time, drive to the black sand beach (Punalulu Beach Park.) The beach itself was small. But there were sea turtles every time I've been there.
Bring a light rain shell and sunscreen. Hilo is the wet side of the island, so it's always raining. Conversely, if you're on Kilauea, you're pretty high up, so the sun is stronger than at sea level.
Hilo itself is a small town. I like it there, but keep that in mind as you plan your outings. Often places close earlier than you might expect.