Peter, I think it's wonderfully romantic and sweet that you want to surprise her.
BUT!
If my husband surprised me with this, and didn't give me any advance warning, I'd be really angry...because I'd want to wear something special and get my hair done.
Not all women are like me, of course, but just think about it. Maybe you could get her to think you are going to a very special dinner out? You've probably already thought about it, I'm sure.
Good luck with all your planning!