If you are interested in ancient history, the museums of Mainz, ancient Mogontiacum, are not to be missed. In fact, the city boasts no less than six museums. The Landesmuseum (State Museum) is easily recognizable because there is a golden horse standing on its roof. It has several departments of art (Judaica, Duch masters, modern painting, medieval religious art, etc.), but the most splendid one is the Steinhalle, which contains hundreds of ancient inscription from Mainz, and some other objects of art made by stone sculptors. Here, you will see the Arch of Dativius, the Jupiter Column, and the Mainz Pedestals. For some other examples, go here.
This museum was visited in 2005.