re: Big Island Hawaii. what is cannot miss to see

Donkeys apparently. Seriously though, we just missed the crazy lava flow happening now. We did the volcano and waterfall helicopter tour out of Hilo, but I heard the boat tour is good too. Not sure what the situation is like now, but worth doing at least one of them. Akaka Falls is on the drive to Hilo, which has a paved path overlooking the falls.
There are a few breweries you can hit like Kona and Big Island.
We were supposed to do the Mauna Kea Summit Adventure Tour, but there was too much snow. Wish we could've done it.
Green Sand Beach: bring tennis shoes if you're walking to it. If you're paying the locals to run you down there, prepare for a bumpy ride. Going early in the day is best, less crowds and cooler.
The best white sand beach we hit, Manini'owali Beach, is across the road from the veterans cemetery just north of the airport.