From a narrative standpoint, regarding event sequence, there are a few fairly obvious examples of episodes Matthew, Mark and Luke could not have relocated beyond the boundaries of John the Baptist's arrest and/or JTB's beheading. Not even if they'd wanted to. (Note: the fourth gospel only alludes to John's arrest and never mentions his death or Herod's involvement.)
The most obvious example - that the arrest (Mt.4:12, Mk.1:14, Lk.4:20) pretty much had to be mentioned before the beheading (Mt.14:10, Mk.6:27, Lk.9:9) - is a clear instance of the general chronologicity that runs through all three Synoptic Gospels. On top of "birth, baptism, cross, resurrection", we must also insert John's arrest and beheading. Besides their chronological sequence, these were 'game changing' moments. Both were very impactful events for the Lord's life and ministry.
The less obvious examples begin with the fact that the Synoptics always agree on locating events [mentioned by at least two of them] either before or after JTB's death. That is, even when Matthew 4-14 diverges so greatly in sequence from Mark 1-6 and Luke 4-9, the jumbled events remain in that same time span. Lots of these are introductory elements - things about Jesus that fit better near the Story's beginning whether the writer's purpose was historical accuracy or just basic narrative construction. And of course, some things mentioned during JTB's imprisonment belong there more obviously than others. (Likewise for content that gets mentioned after John's death, but I'll save that for future posts.)
Three particular instances:
One easy argument is the "old wineskin" episode (Mt.11,Lk.7); obviously, since John's disciples brought questions from John in prison, this event belongs during that time span. One safe argument is all the introductory stuff between Jesus and his own disciples; since Herod kept John in prison for some time, few would doubt that Jesus' recruiting efforts were well underway before John died. One impossible argument is the grain plucking incident (Mt.12,Mk.2,Lk.6); apart from invoking the writers' general agreement on sequence, there's nothing inherent about this event by itself that says the grain plucking couldn't have happened in Judea (and/or) after John died.
In that last case, much more could be said, but it probably boils down to this. If you think JTB's imprisonment was less than one year long, you pretty much have to toss that grain plucking incident into the future. But that does make it odd. On the contrary, if we suppose that the narrative content located in JTB's imprisonment was most likely meant to relate those events which belong to the time span of JTB's imprisonment (and assuming at least one Gospel writer had reasons for knowing so) then that imprisonment must have been more like two years long. Hoehner agreed with this; so do I; Blomberg does not. What do you think?
For more on the chronology of Jesus' ministry, see here and here. For more analysis of Dynamic Events in Gospel Narrative, and how they may or may not imply aspects of chronologicity, stay tuned...