We did this, and the best tip I can give you is, there is a corner in the gift shop, off to the sides, that has seconds (this mean that there is a slight imperfection in the nut or the choclate that covers them, not that there is a health hazard, they are just not pretty!) and they cost like $.50 a bag for Chocolate covered Macadamias, we filled a huge bag, and brought them back and gave to DH's mom and friends, best gifts every....
Kona brewery is awesome, after the tour and tasting, stick around and have lunch there, their pizza's are wonderful!, and make sure you get beer sampler, they put a mat down in front of you and small beers on each place and it explains all the beer to you, very cool indeed, When we were there, the iron man competition was going on, and the were swimming right out in from of the Brewery.
There is a tour here at the Orchid farm, and you should take it too, I think it was cheap $5 maybe... And they ride you around in a tram and tell you what each flower is and then you can explore on your own afterwards, I have some amazing shots, from there, bring a macro lens if you have one.
For the Waterfalls, bring a wide angle lens if you have one, and maybe a 200mm zoom. Very Very cool, and awe inspiring.
This is truly amazing - Volcano national park lots and lots of pictures, you need both the zoom and wide angle.