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There's a string of small coastal resorts, from Poyrazköy just beyond the north Bosphorus bridge, through Anadolufenerli, Riva. Usual beach attractions, but the Black Sea is often rough so beware of the waves. There is also a big golf field, 'Mustafa V. Koç Course'.
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It's as close as the casual visitor to Istanbul gets to the Black Sea. Not much of a vista but it allows you to appreciate the strategic significance of the Bosphorus.
Kavak
A village with an impressive citadel overlooking the Bosphorus and its mouth into the Black Sea. By land it's accessible only by a hairpinning road through the forest, with few buses. It's best reached by ferry from Eminönü (twice daily) or Sariyer (frequent). In the village, a road is signposted up to the hilltop Yoros citadel (a little more than 1 km, 20 min on foot, free admission). There's a pleasant area with cafés by the ferry pier; the place gets crowded at weekends during summer. Nasty big dogs stalk the citadel area at night. See Istanbul#The_classic_Bosphorus_cruise.
The "Polish village" was founded in 1842 by Polish settlers in the wake of the failed uprising against Russian Imperial rule, and reinforced by more settlers after every subsequent civil upheaval until Poland's independence in 1918. Most then returned, but several stayed on, adopting Turkish citizenship but maintaining their Polish language and culture. The village has houses in traditional Polish style, a 1914 little Catholic chapel, and the Church of Matka Boska Częstochowska. It's set in woodland (a nature park) and is a popular day-trip from the city. There's four restaurants and half a dozen small hotels, plus more of each in nearby Beykoz.
Believed to be the tomb of the biblical Joshua, it's the steepest hill in Istanbul which is closest to the sea.