Others may already know this, but here goes. We used our 3 fastpasses. The next one has to be done at a Kiosk. My daughter wanted to ride Splash Mountain. We waited in line for the Kiosk and the CM stated none of the mountains was available. We went to a Kiosk and sure enough, none had any times. So we booked Jungle Cruise. Got out of line, went into the App. There Jungle Cruise was listed. I then used the App to modify my fastpass experience and low and behold, Splash Mountain did have a time I could pick. It was much later, but at least it was there. We picked it and went on.
So the tip is, you can modify your extra fastpass using the App with a lot more freedom than the Kiosk gives you.
So the tip is, you can modify your extra fastpass using the App with a lot more freedom than the Kiosk gives you.