If you want Palo or any of the spas that has to be booked as soon as you get onboard; also if there is a problem with your dinner rotation you'll need to take care of that too. Plus you'll have lunch on board too that you probably won't want to miss.
I found last time there was so many little things to take care of that I missed the sail away party because we got on board so late (Disney transfers and didn't leave the poly till after 11 am.
This time, I'm going to enjoy the whole crusie! Arriving at the port at 10:30 or so; getting onboard; making my Palo reservations; enjoying lunch; getting reacquainted with the ship (It's been 4 years!) enjoying the life boat drill (Hey you HAVE to do it; might was well enjoy it) doing the sail away and then getting ready for an early dinner.
I've taken lots of cruises (Home Lines 3 times, Celebrity 2 times, Holland American 2times and Disney once before) and the best piece of advice anyone ever gave me was one of the cruise directers. He said he saw too many people ruin a good cruise by trying to do it all the first and last day. They miss far too much unpacking and packing, rushing on board, not taking the time to plan the start and end to their trip. I realized I did that all the time, so now I plan my vacation accordingly.