Let's see...after 7 cruises, I'm sure I've got something in my brain worth sharing...lol
On embarkation day, I usually use the time before the Sail Away Party to get acquainted with the ship and do a little hidden mickey hunting. Most of the common areas and lounges will never be as empty as they are on day 1, so that's always been easier for the hidden mickey hunting. I also find it nice to get acquainted with where the lounges/restaurants are when I'm also not trying to navigate around people. I also use the first day to weirdly stare inside the gift shops so I can start planning what I'll buy lol
It's also a great time to check in with guest services/cast members/activity places to see if you can grab a reservation for an activity that you weren't able to book ahead of time. Really wanted some time on the sports deck? Go up there and ask if they have more times available. Brunch at Palo? Tasting? I think they reserve some appointments for on-board booking, so that'll be your chance.
Seasickness happens to someone in my party almost every time I go. Personally, I take a 24 hour Bonine every day just to ward off any potential sea-sickness. Also, I didn't know this on my early cruises, but guest services and the health center usually have a bin of (free) motion sickness medicine nearby. In the case of the health center (pre-covid) I remember it being right outside the door. I'll go and grab a few packets to have on hand in case of emergency. Forever scarred by that one time I got really queasy at Remy and had a really hard time getting myself down to deck 3 and waiting in line at guest services. So! Now I always try to prevent and also grab some early.
Bring a hoodie or sweater or something. It can get chilly inside the boat, especially if you've been in the water outside. I was surprised at how cold I was all the time on my first cruise, so now I'll never be caught by surprise haha.
Not sure how much the app has changed, but I like to check out my activity options for the cruise as early as I can. So I'll go through the app and find all the trivias, towel foldings, jack-jack diaper dashes, anyone can cooks, etc and go ahead and favorite them. This way the app will send me a notification when the time is approaching. I've always found the activities can run back-to-back so I just like to have a little plan on what I intended to do. BUT! I've also totally played it by ear and that worked too.
If you're worried about PAT and you're on the fence about taking Disney Transportation, I would book it. If you're arriving on Disney transportation, they don't care so much what you chose your PAT to be.
Back in the day of the old muster drill, you'd be uncomfortably close with random strangers. I always took this opportunity to strike up conversation, especially with people wearing gold or platinum lanyards. I wanted all their pro tips! No one was ever shy to share anything and were absolutely delightful. I'm sure this would work for any line or activity. Don't be shy. Everyone I've ever talked to on board (with very few exceptions) have been really kind. I got lots of great advice on things to do from them. One of the best being, check an activity out if you think you might be a little interested. Heck, even if you don't think you're interested. They can be a lot of fun. As someone without kids, I made a big frowny face at the idea of Jack-Jack's diaper dash, but it was hilarious and I love it. I can't count how many times I was like "isn't bingo for old people?" Also crazy fun. The adult Match Your Mate should never be missed, even if you think you've seen all the Newlywed game episodes and nothing will surprise you.
Probably my last tip and a pretty shameless one. I've gone to more than my fair share of DVC seminars. They raffle off some onboard credit in there and if I have the time and nothing else is going on, I pop in for a free drinky-drink and listen in on what the plan's going for these days. I did the same thing for port shopping seminars, though I am not as interested in their swag as I am with the OBC. I've never won the OBC from DVC, but the people I've traveled with have gotten swag at the port shopping seminars.