That's not flirting. That is making plans.

Flirting, I couldn't care less. Openly and actively encouraging a situation that is inevitably and clearly designed to lead to compromising your marriage/relationship, is not ok.
I agree. Flirting is not suggesting a "play" on the beach. Unless he really likes The Tempest and there's a production of it near her that they could both watch. Maybe starring Patrick Stewart... although that would go beyond flirting and into "encouraging lust-filled fantasies" for me too.