The city of mainz is indeed very nice for a short visit, as downtown Frankfurt is IMHO not that kind of a highlight (although - if you have nice spring weather an open-air coffee at the main river can be nice too).

You may also consider a short train-trip to Heidelberg (50min per way ==> http://www.bahn.de) or, if you´re a travel-junkie, take the highspeed-train ICE to Cologne (55 min. per way), visit the world-famous cathedral nearby CGN central-station, enjoy a Kölsch and go back.