From a wife who absolutely hates these types of surprises but loves the thought and the process of picking the thing out with my husband, I would not want my bike picked for me. However, it does sound like you wife likes these surprises. If you really feel that you are correct in your read of what she wants then by all means make it a surprise.
However, I really like the peddle idea and going to the shop so she can see if there is something that is there that neither of you had thought you would like but seeing in person/ hands on might be a different story.

And to the people asking about buying online, my read on this topic has nothing to do with buying something unseen, it is about surprising the wife with something as personal as a bike. That is very different than just buying something online when you are the one researching and knowing your measurements and what feels comfortable. At least that is my understanding of this topic.