The intent is for the workouts to be completed over a seven day week and for non-workout days to be rest/recovery days. You can do any activity that doesn’t cause too much additional fatigue. You can go for an easy ride, do some other activity that isn’t too stressful, or take the day off.
If you find the overall training stress isn’t enough you can do additional work on workout days, just keep in mind the intent of the designer. If the workout has a stress score of 80 it’s supposed to be a hard day; if the stress score is 50 it’s designed as a more moderate day.
With that said Week 4 of the 10-12 Week FTP plan is designed as a recovery week so what you do for the remainder of Week 3 doesn’t matter so much.