Everyone is going to have their own opinion of what is a must have/see/do. I did not understand the idea of using a FP+ for a character meet & greet, but due to height restrictions, we did use one for DS3. It was wonderful! Hopped into a line to meet Mickey & Minnie at AK and we were out in 8 minutes. The standby was 30 minutes. We also stood in a few lines that took 20-30 minutes, and "Mr Impatient" survived.
MK: 7DMT (you won't see the interactive games, but you will load and be off to another adventure),
Splash Mountain or BTMRR
AK: Kali River (if it's warm out)
Kilimanjaro Safari (DH stood SB for 40 min while DS3 & I waited for our time. We walked right past his Daddy in line.)
Finding Nemo (maybe) If you want to sit in the pit, use the FP, but not needed. They load the theater quickly and the characters come along the aisles. Folks in the pit have to turn to see the action behind them.
EP: We used it for Soarin. DDs rode Test Track standby, but the line was long when the rest of us went by. This would be your discretion.
The thing about the system, it allows you the chance to not rush in the park and jump in one line while your other really want to do gets a longer line. We were off site and despite the desire to arrive for RD, we ended up arriving an hour after park open. We took our time and got in line where we wanted. We had scheduled most of the FP for later in the day (11am was earliest). No, we didn't get a 4th but it fit our touring style. DDs rode RnRCoaster 3 times in a row--during Star Wars Weekends' final day at noon. Once with FP+, once Single Rider SB and then a CM gave them a

FP for the coaster.
Pretty much, if you WANT to ride it and it will make your family happy, use the FP+. It doesn't fit "normal" plans to use the pass on Nemo & Friends in Epcot, but we did. It worked out well for DH who was in back pain and DS3 to get on, enjoy the break and then climb back on
scooter and stroller.
My one Don't Use would be for Crush. You have an hour report time, but the show starts every 20 minutes. We went through and then we were told we would be waiting for 40 minutes. Our FP was for 12:00. That show was full, as were the next 2 shows. You wait in line (crowd) until the show time. And the FP+ folks are filtered in with everyone else. We all got in and their was plenty of room because they count the number of persons...kids all had a good amount of space around them on the carpet.