Some things to consider.
Certain things (Princess Meet and Greet, Palo, etc.) can be booked ahead of time.
Other things (Changing Dining to early or late seating, Spa Treatments, Alcohol Tastings) can be done on board at specific locations other than Guest Services. It will say exactly where in your Navigator, when you first get to the Port.
Also, Guest Services is really only crowded for a few hours that first day and and then on the final evening/morning. The rest of the time the line is REALLY short. (Like a person or two). Plus, it is staffed 24 hours a day.
So anything you can hold off on and not go to Guest Services that first few hours, I'd suggest waiting.
But if you really need to do it, the line does go pretty fast. While you are in line, you can still soak in the atmosphere. Just resolve yourself to the wait.