Üsküdar is the Asian side's seat of Ottoman power — three Sinan mosques within a kilometre of the ferry pier, the great court architect doing his Asian variations on the themes he was perfecting across the water.
Four and a half kilometres, an afternoon. Start at the iskele; Mihrimah Sultan Camii is directly in front of you, built by Sinan in 1547 for Suleiman's daughter (the legend that he was in love with her gives the building its faint sadness, true or not). Climb the hill to Atik Valide Camii, Sinan's later complex for Nurbanu Sultan — less visited, its full külliye better preserved than Süleymaniye's because tourism never touched it.
Back down to the water for Şemsi Paşa Camii — Sinan's smallest mosque, jewel-sized, built so close to the Bosphorus that birds don't roost on its roof. The folk explanation is one of those Istanbul things you can't verify but want to believe.
End south along the sahil at Salacak. The Maiden's Tower sits on its small islet a hundred metres offshore — best at golden hour, when the tower catches the last light and the European shore is already in shadow. Friday afternoons fill the mosque courtyards; avoid that window if you can.