We did that cruise the same week as you last year and loved it.
Yes, there is a charge for specialty restaurants but I don't recall exactly how much. Seems as though it ranged from a low of $5 (discounted from $10 on formal night) to $20. I think the exact amounts are available on Pricess' web sit but I'm not positive. Reservations are needed and some of the restaurants book up quickly so book early in the cruise.
It seems there was 24 hour dining but I'm not much of a night owl so I couldn't swear to that. I can assure you that you will not go hungry! If nothing else I do know that room service is available 24 hours and is free but remember to tip the waiter upon delivery. If you check your Princess Patter (the daily newsletter) it will give you the hours of each dining area for the upcoming day. The letter is delivered to your room each evening as your steward turns down your bed. It is a source of invaluable information. Princess doesn't do as many P.A. announcements as some cruise lines (thank goodness) so it's up to you to read up on what's happening about the ship.
This was one of our favorite cruises, especially College Fjord. There were a LOT of glaciers calving while we were there and it was just spectacular. Alaska is a place that is truly spiritual in all it's grandeur. We saw fabulous whales near the inlet to Glacier Bay as well as amusing sea otters and other critters. Lots of eagles and orcas on excursions - just fabulous. You've made my morning by recalling those special days. I think we might just have to do that again!