Since this is your "one time trip" (so was mine
) you need more time, 3 extra days is nothing in interior Alaska and will be greatly limited in what you can do. Denali Park will be out in this timeframe, which is the Alaska highlight and a must do in my opinion. If you can get 5 days you could include Denali park and Seward which will at least offer you some fantastic touring opportunities loaded with wildlife and scenery. At all possible do consider it. Going independent with a rental car is the way to go with Alaska RR to/from Seward.
Take a look at some photos, if my words don't impact enough http://community.webshots.com/user/budgetqueen
I do give Seward sailings a big plus and with short time, make the best of it. So I suggest you consider HAL for their Glacier Bay sailings and look for College Fjords included.
Princess sails out of Whittier, possible to transfer Seward/Whittier via shuttle or cab. Carnival, RCI, Celebrity do not go to Glacier Bay, which I consider a must do on an "only" trip. All Alaska ports are worthwhile, but you may want to request visitor guides if you have priorities?
There is no guarantees on weather, and fog can happen anytime. You need to be well prepared in your attire. As for formality- there is plenty of dining available, buffets in the evening off some good food if you don't want the fixed dining- which will require some dress codes. Room service is a good option too. My opinion, food is a very low priority for me in Alaska, I never hesistate to ship meals and last trip only made the dining room 3 times at dinner. Princess does have personal choice which does give an edge in Alaska for the non fixed times. Any trip will be a compromise, the more you know the better, do your homework. Enjoy Alaska!!