These might sound obvious but I'm going to suggest some practical things.
1.) Don't forget your I.D. I know it's like, "DUHHH!" but it's easy to forget it when you're rushing out the door and they won't let you on the plane without it. Also, leave plenty of time to get to the airport - it's not "untoward" to try to be at the airport 2 hours before the scheduled time.
2.) Bring some sort of emergency money and hide it somewhere.
3.) DON'T put anything valuable in your luggage. I once mentioned to my parents when they were going on a trip that they shouldn't put their cell phones in their luggage; but they thought it was a little bit of overkill, "paranoid," etc. One of them had their cell phone stolen from their luggage by the airport workers.
4.) Bring a jacket. It's not
totally unheard of for it to get a little chilly at night in Florida. Maybe an umbrella too, if there's room for it.
5.) Comfy shoes/sneakers. Disney makes for a LONNNNG, fun day and you're gonna be waiting on lines and walking around for much of that.
6.) Suntan lotion, for obvious reasons.
7.) Something to carry your stuff
easily in, again because you're gonna be "on your feet" all day.
Well that's all I can think of for now but have FUN!
P.S. - Oh and don't worry too much about the plane flight, that really should be exciting and fun. Statistically it really is MUCH safer than driving and the Boston to Florida run is an easy cakewalk for these people.