Originally Posted by mauld
Train travel from FRA is very easy-- just remember to get off at the first (Sud?) Mainz stop-- not the Hautbanhof. The train ride takes 10-15 min and it's a local type train. It's a about a 5 min walk from that station. I think you should get RC access as a Diamond, but recently some posts seem to say treatment/ upgrades haven't been all that great. I was there a couple of years ago and was given a large corner room with full river views and a wraparound balcony. It was not on the RC floor, but just below. It's good RC with lots of food/beverages. Not sure how late it stays open though. The hotel is connected to a large shopping/restaurant complex and its about a 15min walk to the main part of town.
Keep in mind that not all of the trains stop at Mainz Sud, the closest station (about a 5 minute walk.) The main station in Mainz would be quite a walk with luggage, but I remember paying about Euro10 for a taxi from there to Hyatt.
Follow the signs to St Stephans, a bit of an uphill walk. Great Chagall stained glass windows. The river walk is an enjoyable route. Otherwise, not too much in town to do.