When are you going? I will be there in April, and hope to do some shore diving.
Snorkeling and shore diving is nice at Two Step (get there early-gets really crowded) I dove there last year with Garry from
http://www.sugar-inc.com/sugar-cottage and we hung out at 40 feet watching hundreds of Spinner Dolphins cavort around us.
My boyfriend does not dive and snorkeled the Manta trip. He was just as thrilled as I was.
This is our favorite beach (for boogie boarding, swimming, etc.) I have seen some people snorkeling there but usually pretty wavy.
Kua Bay (Manini'owali) white sand beach | Big Island | Hawaii
And of course Volcanoes (will be interesting to see what has been obliterated since last year)
Five days is not much time to explore the wonders of the Big Island. We have gone year after year, and have a hard time wanting to leave even after 2 weeks!