You can get the single WiFi package and allow your partner/kids/whoever to hotspot from your phone so you can both/all access the WiFi together.
Excursions are cheaper if you find a local tour company however if you're late back the ship will leave without you. The P&O excursions are more expensive but if you're on one and there's traffic or a delay of some sort the ship will wait.
Although there's a doctor on board I always take a small first aid kit because there's no point paying the excess on your travel insurance to pay the doctors bill if you can solve it yourself:
Paracetamol + ibuprofen
Motion sickness tablets
Diarrhea relief
Dioralyte
Throat sweets
Lemsip
Calpol
Germolene
Plasters
Definitely take motion sickness tablets even if you don't get travel sick normally. I was fully expecting to get sick initially on my first cruise, I get car sick, I struggle on rollercoasters.. turns out I was absolutely fine but my partner who's never got travel/motion sick a day in his life was awful for the first 24 hours until he got used to it.
I always take magnetic hooks with me (you can get these on Amazon), they're fairly heavy duty and all the walls in your cabin are majestic. It was something we could use to hang up raincoats when wet, bags, caps, I take a little linen bag to hang on a hook and wallets/purses/sunglasses/cruises cards/bits and bobs from pockets go in there. You don't get much counter space in the inside rooms so it just meant the surfaces were much less cluttered.
Take little hand sanitizers and use them as often as possible when out on the ship. Wash your hands properly before and after eating.
You can take your own soft drinks on board so if you haven't got a drinks package and want to save cash (especially if you have kids with you) take a few 6 packs of coke or whatever. There's a small fridge in your room. We took enough for a can a day each on port days so when leaving the ship we took a can each in our bags just for while we were out and about. All water in your cabin bathroom is drinkable so you can always take a refillable bottle with you to take off the ship if you don't want to buy a drink at the shore.
We take a hanging organiser (folds flat) for socks and pants. Takes up only a space in the wardrobe rather than multiple people's pants and socks taking up lots of space on the small number of shelves. We also take a type of coat hanger that holds four items at a time. Use them for multiple pairs of leggings or tops. Again, shelf space isn't much if there's a few of you in the room so was just an easy way to save space.
If you have a small child who doesn't like showers you can get a small inflatable paddling pool (if this applies to you I can get you the link of one that fits exactly in the bottom of the shower) we filled it up using the shower and it meant our 6 month old at the time could sit and splash while washing rather than us fighting tears in the shower. There's usually a small washing line (30cm or so) above the shower where you can hang a swimsuit or something but we found it was the perfect place to slot the paddling pool up out of the way when not in use.
Launderettes are free on board and open to use any time unless there's a norovirus outbreak. We took laundry sheets last time in case our youngest had an accident. Didn't want shit covered vests stinking out the room. We didn't have any accidents but did use the laundry sheets on the last day, nipped and got all our stuff washed and dried ready to come home in about 90 minutes while the baby was napping. If you do have small children it's worth taking a small amount of liquid laundry detergent to use in the sink in your room on the small chance the launderettes are closed and your child wets/poos themself. We also used the dryers a couple of times for ten mins to get the swim stuff dry when the kids wanted to go in twice in one day.
We also take a small amount of fairy liquid and a sponge with us to wash spoons or tupperware for any food the kids have had in the room. Our youngest was up at the crack of dawn so they'd have a porridge pouch or something before we went to the buffet a bit later and it was good to wash the spoons as there's no washing facilities on board for you. The tupperware was because we'd take a muffin or a bit of fruit or something from the buffet during the day back to our room in case the kids wanted anything before bed. I know you can leave the room and eat almost whenever but when the kids are washed and in bed with PJ's on it was good to be able to just give them a banana or something before bed instead of going back out.
Sorry I've rambled and if you don't have kids or young kids half of that won't be necessary at all but it might help someone reading anyway.