It is the same price 25.00 a day for one device whether you purchase before cruise or on board that price is for one device at a time so you can share devices but must log off to switch between devices. I would suggest you download the princess app before your cruise you can also make dinner reservations and book excursions and close to your cruise under journey view you can see the daily ship schedule.
Most of the casual dining is no reservations, but I believe that planks and steamers might be reservations once on board I would check on that your first day and it alternates for dinner no lunch as far as I know.
For a late night snack I go to the International cafe it's open 24 hrs and has breakfast items in the morning and small sandwiches quiche soup and pastry if there is no cookies out just ask they usually have them in the back my favorite is peanut butter and chocolate chip cookies should be available.
Yes deck 6 is traditional dining same table server and time choice of early or late. Deck 7 is reservations for times starting at 5pm for a table shared or individual, can also do walk up without a reservation will be different server each night.
Yes all cabins have mini fridges they are empty unless you are elite or in a suite those guests get a one time set up of a mini bar which contains mini liquors, can beer, bottle water and sodas.
Another thing to know is that the reserve dining menu is the same except for one additional item, but I believe that service is better in the reserve section and no need to make reservations.
My Sun princess cruise is November 30 I was able to book dining reservations for the week, but I'm also elite so that may make a difference if trying to book before final payment.
There is no way I could go on a 7 day cruise with a small carry on and back pack or tote bag I just think I have to take more clothes than I really need!😀
I just checked the app this morning and found reservations are now open for the Sun so was able to make reservations for deck 7 same everyday and once on board will cancel for nights I decide to eat elsewhere. O'mallys is listed in the reservation area but can't make reservations I had heard that it is a no reservation casual dining but was hoping reservations may open up since it will be hard to get a table for dining.