just to add--

Waipio Valley-- i'd suggest more than the lookout, do the horseback riding or wagon tour. it's interesting to actually go through the valley.

mantas-- if you don't want to risk the dive (time, money, or nerves). i believe the Orchid Hotel-- the one just north of the Waikaloa hotels) has a night light out to see mantas coming to the shore. we saw 2 in June, and it was worth the 5 min. trip from the Hilton.

Captain Bean's is a good booze cruise with audience participation show. cheesy, but a good value (food, fun, cost) way to see the Kona coast and get out on the ocean, if you don't do any other sailing/snorkling/fishing.

make sure you buy the book "the big island revealed". it's invaluable.