I've stayed at the Hyatt in Mainz in the winter (
PHOTOS)and enjoyed watching the barge traffic go by from their lounge. I did take some city photos on an Easter visit a few years ago and they're
HERE. I walked across the river on that visit and had a beer at a small outdoor beer garden. Gorgeous weather on that visit.
I was in Frankfurt again last week, arriving around 4pm before departing the next morning, and to me it's more of a shopping town. Yes, there's a church or two, and the restored town square, but Mainz just seems to have more to offer when the weather is nice and you want to be outdoors.
Can't really help on dining options.