Rhein - Rhine River
If you visit Mainz in summer it might be a nice idea to go for a stroll at the Rhine River. The best way to get there from the city center is from the city hall I'd say, which is located at the "Brand", next to the Hilton hotel and the "Rheingold Halle". From there you'll already have a great view over the river too (see picture). Then walk in the direction of the Hilton hotel and after a few meters you'll see some red buildings, that's the "Kurfürstliche Schloss" which is listed as being under conservation by UNESCO. Nearby there's also the "Landtag", that's where the parliament of the state of Rheinland-Pfalz has meetings.
The best time of the year to visit Mainz is from June-August/September. If you're lucky, temperatures are about 25-30°C. But it can also rain during some days and get a bit colder, so be prepared. In fall it rains quite often, can be stormy and already pretty cold. In winter, especially in January/February it might snow for some days, but only if you're lucky. Temperatures should be about -5-5/8°C. In spring the weather's just crazy. In May it's often really hot during some days but then it gets colder again. For more information about the weather and which clothes to bring have a look at my Germany page!
This is a really interesting place. The Heiliggeist used to be a church and thus makes for a unique atmosphere.
If you're looking for a cocktail bar you better go to the Havana which is just next door but otherwise the Heiliggeist offers all kinda drinks and dishes.
The age group of their guests is about 25/30+ I'd say.
Central Museum of Roman History
Fantastic Museum with Roman military and civilan equipment, originals and copies of important peices from all over the world. The muesum is right on the Main River, and is one of the few buildings in the area not bombed to rubble in WW2.
St. Stephen's Church - the Chagall Windows
Well, boo on me for not getting any pictures of the phenomenal Marc Chagall windows here, but it's quite difficult for an amateur to do them justice. Trust me, the beauty and poignancy of the stained glass makes this church well-worth a visit, even if you are just in Mainz for one day, don't let the time pass without making a stop here.
Like so many other buildings of all kinds, St. Stephen's church was heavily damaged in World War II; practically demolished. In a spirit of hope and reconcilation and desire for a better future, the parish commissioned Marc Chagall, the great Russian/Jewish/American artist, to design new windows for the reconstructed church. I can't think of any 20th century stained glass - anywhere - that is more effective and more wonderful than that which is here.