We spent Christmas there. Check out our blog if you want at
http://hawaiichristmas.blogspot.com/Be sure to check out state Route 250 from Hawi to Waimea. If the weather is good you get some exceptional views and cover about 4000 feet in elevation gain. Take the Saddle road across to the wet side and Hilo at least once. On the way, be sure to drive at least up to the visitor center on Mauna Kea - up to the observatories at the summit if you can. BTW, kids under 16 are not allowed up.
The active volcano Mauna Loa was a bust for us - poisonous gas closure, but can be good and worth the trip.
The hike out to where the lava goes into the ocean is also spectacular and worth the walk. Be sure to drive past the "entrance forbidden" "no trespassing" and "trespassers subject to arrest" signs on route 130 from the Hilo side. What is really cool is that you get to drive over the lava that has flowed across the road.
we fell in love with the area around Waimea and Waikoloa...my wife wants to move there!
If you want more info, pm me.