Well, based upon your travel style I was going to suggest Kauai/Maui but I see that the Kona flight worked out better for you. You won't get any argument from me - you can't go wrong. On the Big Island we really enjoy snorkeling and I prefer the tours that go to Captain Cook etc over shore snorkeling, but it's basically because I like starting out in deeper water and going into shallow water - a lazy snorkeler. You'll probably like the Kiluea Iki hike at VNP. Hiking is great on Kauai too - it's our favorite hiking island. Here's a link to some hiking info for Kauai: http://www.kauaiexplorer.com/hiking_kauai/ We liked the Pihea trail and the Kuilau trail. Both are fairly easy hikes (I see that they are rated moderate - I thought that the Pihea had "some moments", but Kuilau was pretty easy but not flat).

We once booked a four hour layover in Honolulu and hiked up Diamondhead. Since you have to connect, you may be able to see something interesting in Honolulu during a long layover.