We are AP holders and do plenty of last min trips (some 3-4 days some longer) and usually just book a value hotel for those trips. Typically POP century and often between $70-95/night with pass holder discount. While this is a bit more expensive than some offsite motel 6 that would run a few bucks and night cheaper, all the added perks of Magical Express from the airport (saves about 80-90 in cab fares) as well as negating the need for a rental car, etc make the value resort worth it.
If you think you will spend more than 10 days onsite over the course of the next 12 months, def pick up an AP so you can unlock the better hotel rates, as well as dining and merch discounts. We took it one step further and bought the tables in wonderland for $100 so now we save 20% on our meals. We do a Table Service every night so its paid for itself in the first trip.
When it comes to planning and FP+ etc, you will find that indeed a lot of people do cancel FP last min because they decide to do a different park, kid gets sick, etc so you can sneak in the occasional high demand event last min. We have even booked BOG within 24 hours thanks to the new cancellation cash penalty.
Good luck!
