The Oder promenade is Frankfurt's simplest pleasure: a walk along the river with Poland right across the water. The bridge to Słubice makes the border feel almost casual. One side is Germany, the other is Poland, and the crossing is just part of an ordinary city stroll.
Use the riverfront to connect the old centre with cafes and shops in Słubice. The views back toward Frankfurt are especially good in low evening light, when the church towers and postwar blocks sit together more gently.