It's hard without knowing what your wife is like, but I went from beach cruiser types to lower-end vintage road bike frames in my mid-20s and those were my gateway.
I think my first one was a vintage Schwinn (didn't pay attention to the model back then, but it was from the late 70s) that had bright blue accents. If you want to get her on something lighter, maybe color can be a good point of transition - maintaining some inherent femininity and fun that she might think she's losing with the cruiser shape.