What kind of things are you interested in seeing? There are many near-by towns you could visit that are an hour or less by train from Frankfurt, but it depends on your interests. Heidelberg is a lovely town with lots if things to see and if you are a Mark Twain fan, than it should be on your list. It has more than just the castle to see.
Strasbourg is too far away.
It's at least 2h 10min during normal weekdays and 1h 50min on weekends but always with fast trains for which you have to pay quite a bit. I mean if you are desperate to see Strasbourg for a specific reason ... but your post sounded rather like you're going vaguely for something pretty. Heidelberg also isn't exactly around the corner with regional trains and a bit more expensive than towns nearby.
Lubitsch: surprised you did not recommend Rüd... 😉
Lol. Unfortunately the Rhine towns aren't covered yet here http://denkxweb.denkmalpflege-hessen.de/ in the database of the protected buildings of Hesse. But if you had the book you could see for yourself the difference between Rüdesheim and let's say Limburg where you can see for yourself all the buildings marked in dark red. But technically Rüdesheim even had luck. Usually such a devastating attack on such a small town should have been its end, afetr all the much larger old town of Frankfurt burned down completely. However the buildings along the Oberstraße survived in Rüdesheim and still form a nice ensemble and thankfully the Brömserhof was spared destruction. So Rüdesheim still has some coherent old structures if you know where to look for them and pretty individual buildings it just can't compete with places like Seligenstadt or Limburg.
I only got one day so which one lol. I just want a pretty town with some nice scenary, some old town square, cobbled streets, perhaps a nice bridge and a bit countryside in view etc...spend an afternoon/early evening
They are all great that's the point of it .-). But let's not make it too difficult, usually everybody likes Marburg. Take the regional train there, walk Bahnhofstraße and turn left to Elisabethenstraße. Admire St Elisabeth church, the first Gothic German church, pay the small amount to get into the choir because this is the really interesting part including but not limited to the precious shrine. Then slowly walk up Steinweg admire the houses of which the prettiest are however 19th century and then turn right when you are at the height of the market square. From here on you can roam the streets and peek into the other churches though at some point you should climb the Landgraf Philipp Straße to the castle. Enjoy the view, pay the entrance money for the museum and float through the exhibition rooms which are to the left once you've climbed the stairs but especially don't forget to check out the great hall on the right and the chapel. At some point you should at least walk down to the Old University at Lahntor and if you cross the bridge you'll find that the Weidenhäuser Straße is also very attractive.