Take the trip up Mauna Lea to the Observatory. You aren't far from it, 13,000+ft, 7-8 hour trip. They'll feed ya and give you warm clothes to wear.
Did ya know there is a glacier study on Hawaii?
I spent 3 weeks out there and got back about 3 weeks ago.
I really liked the Big Island.
Go to Hilo and eat at Ken's Pancake House, great food!