Forget the observatory, it's so cold up there that you two would feel everything but romantic. While I agree with Dave about Waipio Valley, you would need to hike down there, and by the time you reach the bottom you will not feel romantic. (You will feel even less romantic if you think of the hike back up!)
If you really want to hike than Pololu Valley at the North end of the island is a better option, it's an easy hike and there is a black sand beach at the bottom.
My recommendation is to go to have lunch at the Four Seasons Hualalai, and after lunch walk to the North end of the property along the ocean. There is a fishpond there in a beautiful natural setting. That's where you should drop on your knees!