If you are in to some science, go to the Keck observatory. Yes, it's an early wake-up, but you are on top of the world
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Don't do the fern grotto. It's a waste of time and money. I was not that impressed by the Luau diner neither. The so called river cruise was nothing more then 2 times 20 min on a barge with 150 people.
We did our own Diving trip. Don't take the early shuttles to the shopping malls as they are not fully open yet. The sun-rise is good, but you need to take the expensive tour in the small group, otherwise you will have paid a lot of money as well to be somewhere in the back rows...
We toured around the island and in the mean time, went to the vulcano national parc. That is the way to do it, because when the ship is in Hilo, everybody goes to the parc, and hence the place is packed.