I wouldn't really plan a whole trip around a day at the volcanoe (but that's just me). Having been to the volcanoe on two separate occasions, it wasn't the highlight of either trip.

Also, for people who have never been to Hawaii, one reason that Maui, Kauai or Oahu are nice for the first time is that they have LOTS of great sandy beaches. The Big Island doesn't have that many (although the Hapuna Prince Hotel has a stunning beach).

As for the Hapuna Prince vs the Hilton -- do a search in this forum or in the Hilton section. There are lots and lots of discussions about which hotel to choose on the Big Island. My vote would not be for the Hilton -- but other people like it.

It really depends (besides the volcanoe) what their interests are.

Good luck