Frankfurt an der Oder is the smaller, eastern Frankfurt: a Brandenburg city on the Oder River, directly facing Słubice in Poland. It is not a polished postcard town, and wartime destruction left gaps in the centre, but the border setting gives it a character that is different from the towns around Berlin.
The best visit is a slow walk between the Marienkirche, the Kleist Museum, the riverfront, and the bridge into Słubice. In an hour you can move from Brick Gothic Brandenburg to Polish cafes across the Oder, with university buildings and reconstructed civic monuments along the way.
Frankfurt works well as a day trip from Berlin by regional train. It is worth going if you like border cities, literary history, and places that show their scars rather than hiding them.