I think a lot of the success or failure of an "unplanned" trip is deep within your (and your family)'s need for "perfect". I personally really enjoy the planning.....maybe it fulfills my fantasy need to think I'm already there....but I like the process, I like making changes as new things occur to us, or in the case of things like Anna/Elsa, they become possible (no way would I stand in line for an hour, let alone 6+ to spend 5 minutes with a character, but with the FP+ we have more than one stop planned now).
If your personality allows you to take whatever hotel has space (or find an off site condo if all onsite are out of your price range), then last minute is no big deal. If you can live with whatever restaurants you can get an ADR, and not be disappointed that your favorite doesn't have availability, no problem. And it appears that even offsite staying newbies are able to get FP+ for almost everything while they're in the park.
I'd say that if you can be flexible and just be utterly happy that you are at Disney and not at work.....you'll have a great time!
Since it will be a short trip, maybe try something you never have....stay at a higher level hotel (or lower if you usually stay deluxe) or try staying offsite. If you usually are there at rope drop, try sleeping in and staying to closing (or the opposite from your norm). If you're usually a commando for the "E" rides, try taking it slow and spending more time looking around at the buildings, the little character interactions (I could spend all day on Main Street, lol)......or if you normally go slow, try racing from one big ride to the next and see how many of them you can do in one day. Stay late and watch the "Goodnight kiss" (if you don't know what that is, google it).
We live on the West Coast so our trips are usually long and well planned, but twice we have had a last minute chance to go to Florida and we believe it is illegal to land in the State of Florida and not at least go to Disney for a day or two, lol. Our shortest was we found out on Wednesday and we were in Disney on Saturday and Sunday, and away on business for Monday and home the following Thursday. A whirlwind for sure, but we did almost the exact opposite of how we normally tour, and it was a blast! Exhausting and hilarious, but it was fun. And we learned that there are some things we liked and kept in our next long trip, and some things we vowed never to do again, and one thing to never speak of again, lol.
The worst day at Disney is still better than the best day at work.
