Lava flow wasn't going while I was there, and the crater by the museum had caved in 50 feet below where the old surface used to be. Also was emitting so much gas they closed the road that drives around. Keep an eye on this site
http://www.nps.gov/havo/planyourvisit/lava2.htm
Also got screwed visiting the peak since the locals were protesting the building of a new telescope, they shut down the road
Make sure to plan a trip up to waipio valley, its a bit of a drive from Kona but its gorgeous. Its where the ending of water world was filmed.
I wasn't overly impressed with the green sand beach, it took a long time to get out there and you will need a higher clearance vehicle. Its a tiny beach and while we were there it was packed with people.
If you plan to go to any of the black sand beaches and swim make sure you have some form of foot protection. While the beach is fine, not too far into the water the rocks were quite coarse, at least at the few we visited. I'd also plan having foot protection period as I stepped on a sea urchin, never really ran into them in Oahu and Kauai, but saw lots of them on the Big Island.