In Warsaw there are nine bridges crossing the Wisla (or Vistula) river. Not all of them are for pedestrians or cyclists, but the ones that are I strongly recommend you visit. Warsaw looks amazing when you’re looking at it from those viewpoints. What I can suggest is to make a bicycle trip, like Vistula tour, which will take you across all those bridges.
The start could be at Siekierkowski bridge (check out the article) and if you will be heading north there are very nice roads on both sides. For me, the best idea is to move on one side of the river then turn around at Most Gdański and go back on the other to see what the riverbank can offer you later on in the day, because you will find lots of spots to hang out during the night there.