The lava is hit or miss. When I was there it was closed off.

Craters Nat'l Park (seperate from the volcano).
If you're kind of eccenteric like me, there's Volcano Winery (the southern-most winery in the US!) near the southernmost point of the island (about 2-3 hr drive south of Kona). There's a coffee plantation (I forget the name) about 30 minutes south of Kona. On the east side (Hilo), there's the Mauna Loa Macadamia nut plantation that gives free tours. Also the Pacific Tsunami museum. Mauna Kea, the mountain/volcano, can be camped. Or, you can drive up it towards dusk and catch a show at the ranger station; they bring out some pretty decent telescopes and do an astronomy show. Dress warm; you're probably 5-6000 feet up! If you want to do this, check it out first, they might require a 4x4 car. I think you can also drive to the top and see the actual observatories.

Driving the island is actually quite big, so prepare for it if you want to get around it all!