You can book excursions on the pier when you arrive and it generally a few $ cheaper. Often the same tour operator. A WORD OF CAUTION…DON’T BE LATE GETTING BACK IF YOU DO. A ship book excursion that’s late they will wait for. Not the case if you book on your own. More of a concern on all day excursions than morning or early afternoon.
US Dollars are gladly accepted generally 1:1. I like to get change in local currency (yeah they look at ya funny) but it’s usually a colorful keepsake.
Night before vs day of…personal preference. I’ve done both and I really like the extra night but if there are plenty of flight options I’ll take an early flight knowing that there are other options to still get there before departure.
You want to get on the ship a soon as possible the departure day. Gives you time to wander around and get a feel of where everything is.
Feel free to ask all the questions you have.
Oh, yeah. Leaving out of Port Canaveral I recommend Quicksilver - assuming you are flying into MCO. Really not much more than DCL bus and a lot less hassle.