What's the budget for accommodations? Hotel or condo or home?
You MUST do the night swim with the manta rays
Flum'in Kohala is a lot of fun
http://fluminkohala.com/
Buy the book "Hawaii Revealed" now for planning excursions and island info - this is an absolute must!
The road to the sea is a hoot, but you MUST have a 4 wheel jeep.
The most southern point of the USA, you can jump off the cliff into the ocean here.
ULU Ocean grill at the Four Seasons - for breakfast buffet and a quintessential Hawaii paradise view (and great food)
http://www.fourseasons.com/hualalai/dining/restaurants/ulu_ocean_grill/
The chocolate tour
http://www.ohcf.us/ is fun as is the vanilla tour
http://www.hawaiianvanilla.com/
Tex Drive in for Malasada's
http://www.texdriveinhawaii.com/
Waipio Valley is very cool. MUST have a 4 wheel jeep to go down the road into the valley. Bring a lunch, eat it on the beach. Don't go into the water.
The big island is aptly named. You WILL spend a lot of time in your car.
There are some very good local restaurants in and around Kona.
Love the big island!