I was there two years ago this past spring. We were there for a wedding and had limited time on the island.so we didn't see the summit. We did go down to the volcano park and that was pretty cool, I would recommend that.
http://www.nps.gov/havo/index.htm
Found a great snorkeling spot just north of Waikoloa (where we stayed). It is called Beach 69 and not a lot of tourists would go there compared to Hapuna Beach (just north of Beach 69).