Here is a list of things that you would want to prebook at your check in day:
1. Elsa and Anna meet and greet
2. Princess meet and greet
3. Disney Jr. Breakfast
4. BBB (extra cost)
5. Reserve sports simulators
6. Princess Tea (extra cost)
7. Palo and or Remy dining
8. Excursions can fill up quickly for the desired times. For example, they might have 2 times of day for an excursion...one could be at 7:45 AM and the other might be at 11:45 AM. Our family would prefer the later time slot because...well... we are on vacation and don't want to wake up early to go on an excursion.
9. Cabana at
Castaway Cay. These are typically filled up by concierge and platinum cruisers as they can book 120 in advance. But you might get one if you put your name on a wait list once onboard.
Now, if you can get an earlier Port arrival time (PAT) and get on the ship sooner ( your boarding group number is based on your PAT) and get those desired items listed above. They do tend to hold some in reserve for guests to sign up for in case they missed it during their online check in.

For example, a PAT of 10:30 AM would most likely get you on the ship by 11:30 to 11:45 and you can make a beeline to sign up for those items mentioned.