Cabins - decide if you want all inside, all outside, all balconies, etc. Cost difference is significant. Also, whether you want connecting cabins (2 set, 2 sets). We happen to like rooms aft of rear elevators. Others say midships and lower has less movement if that's an issue.
Dining - Make sure your reservations are linked so you dine together Need to decide whether to do early dining (~5:00 ish) or late dining (~8:00 ish). In case you don't know - you have an assigned dining rotation and table in the 3 main dining rooms. Your servers rotate with you so they get to know you and what you like. There are also two specialty restaurants for adults only, pay extra, reservation required, better dress required. And several quick serve options. For breakfast and lunch there is a buffet or some of the quick serve options.
Excursions - you've cruised so you know about them.
Most everything else (shows, movies, character meets, family games, trivia, cooking demos, etc.) are free and first come.
There's a bunch of liquor tastings that you can reserve but as a first cruiser they will likely be all taken by the time it's your turn.
You can't do it all and (IMHO ) trying to plan too much just causes stress. Just let it happen and rejoice when it does.