I love doing things like this. I say leave it as a surprise.

My ex (waaah!) thought he was flying LH Economy NYC-Barcelona while I was in Biz, and for various reasons he checked in well before I got to the airport. Waited in Economy line and when he got to the desk, the agent said "you know you're in First right?"

He thought First = Business, so when we actually experienced LH F, he was amazed. FRA FCT, showers, Merc transfers, caviar. Still talks about it to this day. And despite the 'ex' status, it was miles well spent !