If you're interested in local churches head to Altstadt Mainz— home to Mainz Cathedral. You can also discover the museums and cathedral in the area— your sightseeing tour starts here.
If museums are on your list of things to see, you might want to browse the collections at Gutenberg Museum in Altstadt Mainz. If you want to see more of the museums in Mainz City Center, you can easily walk to Naturhistorisches Museum or Landesmuseum Mainz.