Tiber, or Tevere, or flavus(the blond, because the yellowish tones) is a good choice of stroll in any season. Great view in the sunset, with St. Paul Cathedral’s profile on the spectacular Roman sky or the angels from Ponte Sant’Angelo. You may see Isola Tiberina, shaped as a giant boat and do some birding. You may have a bike ride or a photographic stroll on the bord of the water; just go down the stairs you find near every bridge(careful, slippery and sometimes stinking). Btw there are around 30 bridges going from very romantic and picturesque to the very prosaic or the very modern arch of Ponte de la Musica. In late summer the Estate Romana shelters number of pizzerie, lounges, shops and shows near the freshness of the water. The best grattachecca( Roman version of granita)you find at Ponte Milvio. Enjoy!