The Big Island has a ton of things to see. The Hilo side is very wet and rainforest like, while the volcano and Kona side is VERY desert like. I stayed at a B&B right in the town of Volcano so had a ton of time to hike around. The chain of craters road has changed each time I went there where fresh lava flows reclaim more stretches of it. I have not checked recently, but if you are fit enough, you can hike out to active lava if its running good. You will need plenty of water and a couple of flashlights.
Other areas around there are the Thurston Lava Tube, Volcano House... (get a bite to eat and sit at the top and overlook the Kilauea crater. If you are dressed for it you can go stargazing at Mauna Loa observatory... Pretty cold up there at night!! Overall, the drive around the main Kilauea caldera is worth it. Stop and look at Halema'uma'u crater as well. It's a crater within a crater!! Take PLENTY of FILM!