What day of the week (that makes considerable difference)?
I would take the S-bahn in the opposite direction to either Mainz or Wiesbaden, these are charming small towns on the Rhine (Weisbaden is more upscale and touristy, Mainz is more of a working town) and much more pleasant than the big city of Frankfurt. They have more than enough cultural attractions (e.g. Mainz has a museum to Gutenburg as he was from there, Weisbaden has the historic casino), but more importantly great beer and decent food. More pleasant walking than in the big city. Unless you have something specific that you want to see in Frankfurt, this is the better way to go.