We had an overnight in FRA a few years ago and went to Mainz to stay for the same reason you're contemplating.....a nice walk and view and a really good dinner. The train trip was pretty easy each way for us. Got up early and headed back to FRA, only because we were flying 1st class home so got to to into the first class terminal and wanted to enjoy the experience. I'd like to get back to Mainz sometime and explore a bit more.