Much Ado About Murder - S8-E20
Corrected entry: After learning it's her turn to plan date night with Castle, Beckett confides to Ryan that she's drawing a blank as to outdo what Castle planned the last time and needs help brainstorming ideas. As an intelligent woman who knows Castle as well as she does, and considering that in season five's "The Lives of Others", she came up with, planned out, and executed an elaborate "murder mystery" for him to solve, planning something just as big and embellished, even off the cuff, shouldn't be as difficult for her as it's presented to be.
Correction: Or maybe what she meant was that she was conflicted on how to outdo that stage. She's not stupid.