My favorite thing about walking next to the bank of the Seine in the center of Paris is to visit the booksellers' stands and browse their second-hand stocks. You can find them on both sides of the river.
The Parisian bouquinistes' history goes back to the 16th century. They started to fix the boxes you can see in 1930 and they are part of the UNESCO World Heritage site of the Seine.
Opening hours: from sunrise to sunset.