I would also highly recommend Pinnacle Grill. Make your reservation quickly after boarding, as they fill up quickly. They will be able to tell you when the formal nights are so that you can avoid them. Our first visit years ago, however, was a formal night, and it was fine, although I prefer the main dining room on formal night.
It was also a lovely treat to have lunch in Pinnacle Grill. We did this on the day we visited Glacier Bay. It was a nice atmosphere to spend an hour or so, in addition to our time on deck. Of course, you won't hear the glaciers calve from there, but the view is still amazing. No matter what the day, I'd still recommend lunch in Pinnacle Grill. I believe it was $10 pp. Dinner was outstanding, also.