Kona is an absolutely awesome place -- especially compared to Honolulu, or really anything on Oahu.

I wish I could recommend renting a 4WD vehicle and going to the top of Mauna Kea, but unfortunately there's just not a whole lot to do up there -- you can look in on a couple of the telescopes, and drive around the top between all of the telescopes, but that's really about it. It's not as impressive as it really ought to be.

I'd definitely recommend driving around the island, though; it's a good way to spend a day.

Hope you two have a good time! :)