I arrived at the port at 10:30 am for a 7 day cruise. Even though we had to wait till noon to board, I was near the head of the line and it went very quickly once boarding started. By noon, the line was incredibly long and I was glad I got there early.
We made a mad dash for Palo's ressies and were among the first ones there, same for the spa and got the times that we wanted for both.
On the Space Center, it is quite a large place and lots to see. You could spend a lot of time there and I'm just wondering if there will be enough time to see everything without feeling rushed. It's also not cheap, $31 for adults and $21 for children.
Just getting on/off the bus to get to the different venues, the IMAX movie, the incredible gift shop, a few hours might not be enough time to do it justice.
They closed the most direct route from the port to the general public for security reasons and you have to go around a longer way to get there. I went there after the cruise and I had to turn back and take the long way.
Hope this info helps.