16 day panama canal

Going to be taking our first long cruise in October. Longest to date has been a 5 day. Any tips for making this long voyage as co.fortable and fun as possible? Also a question: we are thinking of buying a mattress topper to bring along. Has anyone done this? We are thinking the extra $100 would be a big boost to the comfort of the beds. Anyone know if it's allowed? Cruise leaves Seattle and ends Mobile

Thanks for any advice!!!