I'm also curious what my expectations should be as a first time cruiser on Disney since we will have the last opportunity to sign up for things. I am aware that we can't sign up for the excursions until 75 days prior.
Since you're on a 3 or 4-day cruise, I assume, given that it's the Wish, there are a lot of activities that you can do that do not require signing up for anything or any additional cost. The only two ports are Nassau and
Castaway Cay. If you've never been to Nassau, you might want to look into excursions, but I don't imagine they'd be sold out since a lot of people stay on board at this port; you might also just want to get off and either do your own thing or wander around the new port area or into the town or find a way to a beach if that's your thing.
Castaway Cay, there are a lot of things you can book, but a lot don't really require signing up in advance - you can do a lot of them on the day you get there (bicycles, snorkelling, etc.) or just go to the beach or swim or walk around the island through the pathways or shop. And eat. You can book excursions like parasailing (did that once; had it cancelled three times due to wind), kayaking and walking tour (did that twice and really enjoyed it) or other excursions but you can enjoy Castaway Cay without spending a penny.
I always recommend that people not do Palo for their first cruise. Yes, it's nice (although I'm not that thrilled with the changes they made to Palo on the Wish), but then you potentially miss one of your dining rotations, and you really need to experience them all. We went to Palo on our first cruise (Wonder) and missed Triton's. We've never sailed the Wonder since, and I do regret missing that restaurant a little, just to experience it.
Make sure to download the Navigator app as it will tell you all about what's going on during the day. There's trivia, live musicians in some places, different crafts, character meetings, activities in the Hero Zone, movies, and the live theatre show every evening, etc. Not as much going on as on the longer cruises, but we still never seem to have time to do it all if we also want to experience the adults-only area, swim, Aqua Mouse, beignets in the Bayou, shopping on board, pirate night/fireworks, time in other lounges, enjoy just relaxing time sitting on deck or our verandah.
I've been searching to boards and seeing things about signing up for cabanas at Castaway Cay.
You'll not likely get any chance at these. Concierge guests get first crack at them and there are so many concierge guests that not even all that want them get them.
Is it worth it to get magic bands?
I don't think so - you can only use them to open your room and charge to your room. You still need a key to the world card to get on and off the ship so I don't see how it makes it any easier. If you already have Magic Band + you might as well bring it and connect it but I don't yet see the utility of buying one specifically for the cruise.
What time frame are first time cruisers usually given to board the ship?
As a PP mentioned, you'll pick a PAT (port arrival time) once you've done the check-in at 30 days. They
can restrict you from entering the terminal until your PAT and certainly did do that through the worst part of covid and can if there are too many people (there are fire codes that they have to maintain that restrict the total number of people in the terminal at any one time) but it seems to be more relaxed now. Your PAT will be tied to a boarding group number and when that number is called, you can board the ship. So the later your PAT, the higher your boarding group number. In general, open boarding (when any boarding group can board) is before 1 pm, depending on how busy the ship is and how early they start. So even if you have a PAT of after 1 pm, you'll likely get into the terminal before 12:30 pm and on the ship shortly after. Be aware, though, that rooms are not ready until about 2 pm so you have to carry your carry on bags around until then which I find to be a pain. If you get to the terminal earlier than your PAT and are let in, you'll have to sit around the terminal until your boarding group is called or open boarding is called.
The main dining room that is open for a sit-down lunch is usually open until at least 1:30 I think, the buffet until a bit later and there are the pool deck places for food as well. So you don't need to rush to get on board to eat.