As a wife, I can say that we do not need to be tricked. Honestly, this doesn't have to be a top-secret operation. I'm sure she spends money on stuff. You spend money on stuff. There's no need for all of the secrecy. Or, maybe there is. But most likely, no. 
It's cool. I just figured I'd had WAY too much wine.

When your wife realizes that every room is full of tanks, just tell her, "Baby, I'm just smooth!"