If there wasn’t enough to do in Istanbul itself, the first and second capitals of the Ottoman Empire (Constantinople / Istanbul was the third) are easily accessible by public transport as worthwhile day trips.
Three hours from Istanbul by tram, subway and bus, Edirne is a relatively small but fascinating place. It’s home to the stunning Eski Camii mosque, built by the famous Ottoman architect Suleiman, with red arches reminiscent of the Mesquite in Cordoba. Across the river lies the remains of Roman Baths, and an Ottoman Hospital.
Three hours from Istanbul by fast ferry and subway, Bursa is Turkey’s second city, a huge, vibrant city of 2 million people. It is home to the famous Green Mosque and intriguing graves of former sultans.