I agree with GrannieAnnie. Get your wife's input. Give her a homemade gift certificate then let her pick it out.
I remember one of my cousins was really upset at her husband when he surprised her with a new car. It was a stick shift and she only knew how to drive an automatic. It stalled out on her in an intersection and she got out of it and refused to drive it ever again. I know - extreme case but you get the idea.
I also agree that you are a sweet heart for wanting to get her a lighter machine.