I suggest a 5-day. Three days usually means you spend the entire time on board. With a 5 day you get land excursions for two days, usually, where you can debark for several hours each day and sight-see, or enjoy a beach, or go shopping. Some have "swim with the stingrays" excursions (and similar) which are awesome for the water-adventure crowd. One year we sailed to Jamaica and during our land time we took a banana plantation tour. Another year we sailed to Tulum, Mexico and spent the day at the ruins soaking up the history and the sun.
For motion sickness, there are now ear magnets that reduce or completely eliminate that. My mother in law was queasy her first day, bought the magnets at one of the ship's stores as recommended by her doctor, and she was fine the entire rest of the trip and had a wonderful time.
More recommendations: bring your cell phone and keep it charged, so you can take lots of pictures. You'll save a fortune on professional photos on board (the ship photography crew is everywhere).
Do -not- get suckered into buying any of those "limited edition serigraphs" at the art auction. They aren't worth what you pay for, and they charge $50 for ground shipping even if you get one of them for only $5. You don't find this out until after you've already been awarded the bid and they ask you for shipping info.
DO explore the ship, as much of it as possible. If you prefer to spend most of your time "adulting" then usually the bow (front) of the ship has a 21-or-older-only area, with small soaking pools and jacuzzis and hammocks and a bar, where adults can lounge around and relax without a bunch of toddlers running around.
If you want to hang with the grandkids, it's the stern-end (back) of midships, where you might even find a bumper-car area, playground, family pool and kiddie pool, and lots of music and games.
There are also cruises with ships designed specifically with families and kids in mind (Carnival and Celebrity have some), with day-camps so the adults can spend time with each other for several hours every day, and the kids can have lots of activities.
Another DO: get the best stateroom you can afford. The only people who say "it doesn't matter, you never spend time in your room anyway" are people who get lower inside cabins with no windows. If you can get a room with a balcony, that does -not- have a limited view, get it. Doesn't have to be an owner's suite, and most balcony rooms are affordable these days.
Another DON'T: don't get the booze package. In order to get your money's worth, you have to have around 5 booze drinks every single day, per person per room. That is a LOT of booze.
A DO: do buy a 6-pack of water to be brought to your stateroom, when you make your reservation. It'll be cheaper than buying it once you're on board and the tap water on cruise ships doesn't taste good.
Another DO: bring 1 bottle of wine, per person, on board if you drink wine. You're allowed, and most rooms have mini-refrigerators these days, so you can have a glass every evening before dinner if you're into it, without paying $12 per glass.
A DO: if the ship comes to port and you're able to debark during the trip - then do so. Even if you'd been to the same place for 10 cruises in a row, it's just really nice to get off the ship and walk around.
A DO: bring your passport. Even if you're only going somewhere within the USA, it's good to carry it around with you. This leads me to another DO:
Have a SECURE spot to carry your passport and bring it with you if you debark the ship, again - even if you're just going to another part of the USA. You probably won't ever need to show it to anyone but it's one of those "just in case" situations given the current political climate.
A DO: do bring comfortable walking shoes, preferably slip-resistant, and use them whenever you leave the carpeted safety of the inner ship. Deck shoes, yes. 3-inch-heels around the the outside deck, no.
DO: splurge at least once on their high-end restaurant, which is never included in the "anytime, anywhere" dining plan.
DO: enjoy breakfast every single day, but remember to not overeat in the morning. It's VERY easy to do when you can have all the bagels and lox and bacon and biscuits and pancakes and pastries and eggs and grits you can eat. Balance your need for indulgence with your need to be physically capable of movement all day long, and save dessert for after dinner.
DO: enjoy shopping on board but do a little research in advance. If you like alcoholic beverages, find out how much they cost near us at the Villages first. When you get on board, compare costs with what they have at their duty-free shop. There are definitely some amazing deals but some of them aren't good enough for the extra time you spend receiving your purchase at the end of the trip.
Finally - we end where we begin: take LOTS of pictures! If you choose the wifi package you can upload them to the cloud, and not have to worry about maxing the memory on your device. You can weed out the ones you don't like when you get home. You will not be able to access your data plan on the ship and on most ships, wifi is not free, or particularly fast.
Not sure if this will let you see it but this is our ship going under a bridge, on one of our cruises in 2015.
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