The big island no ka oi!
+1 for Waipio
You can also go horse back riding along the rim of the valley
You can experience the mantas for free. From just after sundown till 1-2am the mantas feed directly in front of the Sheraton Keauhou resort (the resort pumps plankton out a tube). You can go to Keauho bay, walk around to the beach on the bay side of the hotel and swim/snorkel/scube around the corner to the ocean side of the hotel, there are spotlights on the water so the mantas are easily seen.
Have dinner at the Four Seasons Pahu i`a restaurant
blue cheese macadamia nut crusted Prime Dry-Aged New York Steak .. deeeeelicious!
Where are you staying?