Each kid has a guaranteed spot in their age appropriate group--you don't need to do this at all tonight. Come back and do that part in a day or so.
First timer--you won't get a cabana. Find something else to do with your $500. Check back from time to time as people do cancel, but it is really not likely. That said, I probably did 20 visits to
Castaway Cay before the cabanas existed, and it is great. A cabana is nice, but not needed to enjoy your day.
Adult dining--1/2 of the reservations are held for on board booking. If you can get what you want, great. If not, you can try again on board.
Spa--unless you have you heart set on a specific service at a specific time, also not an issue--you can book on board if you don't get it on line. There are also specials for before noon on the second day, port days, etc. that you can't get on line.
Excursions--these sell out. Get what you want. They don't "hold back" any spots, so grab now.