Istanbul Province basically extends over two peninsulas surrounded by Black Sea, Bosphorus, and the Sea of Marmara—Çatalca to the west of Bosphorus on European mainland and Kocaeli to the east of Bosphorus on Asian mainland.
Since 2005, Istanbul Province is officially co-terminous with the city of Istanbul, as the city borders were extended to include everything within provincial borders, but whatever official designations say, other than the huge metropolitan area—in a triangular shape, which has its base on the Marmara coast, covering an area up to 25-30 km long from the southern mouth of Bosphorus at each side, with the height of the triangle going all the way to Black Sea along the Bosphorus—the rest of the province is rural, or at least suburban, in character.
On Çatalca Peninsula, geographically an extension of Thrace, a continuous conurbation formed by summer houses of Istanbulites—concrete cottages in usually densely packed, albeit somewhat leafy, housing estates, which people of the crowded city flee in every possible opportunity, which makes highways west of city highly congested on Sunday evenings in summertime—lines the southwestern coasts along Marmara. Inland is mostly open farmlands producing much wheat and sunflower, and dotted by villages, although landscapes get more industrial as you get closer to the major highways or the outskirts of Istanbul. The vegetation gets lusher as you approach Black Sea coast, although some of the forests close to the shore are pierced by ugly open pit-mines. Another feauture along Çatalca's Black Sea coast is quite large Lake Terkos (Terkos Gölü, a.k.a. Durusu Gölü), a freshwater lake although seperated from brackish Black Sea only by a series of dunes and one of the major sources of drinking water of Istanbul.
On the Marmara coast of Kocaeli Peninsula, the city of Istanbul proper well extends to (and beyond) the provincial border. Inland of this peninsula is more verdant than Çatalca, with some of the hills (around Alemdağ) covered by heathlands, a rare habitat that is found only in a handful of locations across the World naturally. The Black Sea coast of this half of the province is also wooded, but again just like its counterpart to west, is cut through by open-pit mines at several locations.
Virtually among all options of getting into the province, you will have to touch down Istanbul first in one way or another, although it is generally possible to get off the buses heading for Istanbul from locations in Eastern Thrace in Silivri, around 60 km west of Istanbul.
Due to the geography of the province lying elongatedly on peninsulas, major routes follow a west-east axis.
The highway D100 which closely follows the coast of Marmara on both sides of Bosphorus, and the motorway/toll-road O-3/E80 which lies in parallel with D100 a few km north to inland, are the main backbones of the traffic in the province (and also the main roads connecting it with neighbouring regions), turning into a congested urban road, especially in the case of D100, when crossing the city of Istanbul proper.
D020 and D010 (which is essentially a continuation of D020 through Belgrad Forest), lie further north of O-3/E80, mostly traversing wooded rural areas of northern parts of the province, on both sides.
A few daily trains from Sirkeci Station connect a number of remote inland villages in addition to the town of Çatalca to Istanbul. Other than the line in the city of Istanbul itself, which lies along the Marmara coast, there is no railtrack on Kocaeli side, so your option of getting around by train there is pretty much limited to suburban trains plying between Haydarpaşa Station and Gebze out of provincial borders.
While the alternatives for your next destination is limitless thanks to flights from Istanbul's Atatürk Airport to all six inhabited continents, here are some of the closer destinations: