Definately spend the night on the Queen Mary - it was a wonderful treat and very informative. The only thing that disappointed me was no ghostly visitations - and I waited all night lol.
For early check in you have to take your luggage down to the lower area where the museum is - you check in, drop your bags then you can wander around a bit. But you wtill have to get in line to board - and the line ups were long. Dh stood in line while I wandered through the shops right at the Carnival dock. The Pride was docked on the right (starboard) side.