My last minute trips:
DW and I several times have been at DTD for the day and decided we wanted to stay the night. Called the resorts directly (back before MS was open on Sundays) or MS and checked in within 15 minutes.
At MK for the day in Feb ’14 for our anniversary, called MS @09:00, and booked a room at BLT for that night. Checked-in @11:00 and the room was ready. We did not plan on this day going like this until @05:00 while still in bed.