I've checked in around 12:00, even though my document said 2:00 check-in. My family likes getting on early, before the crowd. You can walk around the ship and get familiar with where everything is located. As mentioned, you can have lunch in the Windjammer. Before you know it, your luggage has arrived at your cabin.
Getting to the ship early will get you through the check-in line fast. I recommend doing your check-in on-line.