Never ask your wife to try to guess what present you are getting her, she will figure it out and it will be disappointing for both of you.
I think I give pretty good gifts, but my wife likes to try to figure out what I am getting her ahead of time. Normally she is pretty good at it, but leading up to her birthday I really had her stumped. This made me pretty smug so I kept teasing her about not knowing what I was getting her.
Finally she got frustrated and I was so confident that I gave her a hint. This lead to her guessing it and ruining the surprise.
She was disappointed because she guessed it, I was disappointed because I lost the opportunity to surprise her with a cool gift, but I guess it does not really matter because I did not end up getting her that gift anyway.