What are some cruise essentials?

we are going on a 4 night 5 day mexican riviera on carnival cruise lines. what would be some items i should bring. and also, my little bro is complaining abou cruise length, we were goin on a 7 nighter, but it ended up not fitting in the budget, i know i kinda feel the same but, i mean, IM GOING ON A CRUISE!!! how can i get him to stop whining i know it makes my mom feel bad.