West Berlin was established as a political entity under the terms of the Potsdam agreement of 1945, and occupied by the four allied powers in a manner that mirrored the division of Germany as a whole after the war – that is, it was divided into four zones of occupation, each of which was controlled by one of the four powers. The Soviet zone effectively amounted to the eastern half of the city, while the Americans, British and French each had a smaller zone to the west.

Postdam allowed for three narrow "corridors" which stretched from West Germany to the city and allowed it to be supplied by air, by rail or by road. West Berliners were permitted to use these corridors to travel to and from West Germany, which was about 100 miles away by road – however, it was a condition that vehicles not stop at any point along the road, so while it was perfectly possible, and quite normal, to make the journey occasionally, in practice leaving the city to "go to the countryside" was something that needed to be planned and prepared for, rather than a spontaneous activity. Allowing for passport checks, the journey also took around a couple of hours each way.

There were of course also a few semi-rural areas inside West Berlin which were popular spots for getting away from it all for a few hours, of which the Grunewald, a pretty substantial forest area in the south-west of the city, was by far the largest and most popular.

The Berlin Airlift of 1948, which you reference, was the product of a Soviet attempt to force the allies out of West Berlin, and began with the cutting off of the land-based road and rail corridors. The western allies organised delivery of all necessary supplies via air, gambling – successfully – that the Soviets would not risk escalating the situation further by actually shooting down aircraft using the air corridor; it's worth pointing out that this air access was guaranteed under the terms of the Potsdam agreement, while the road and rail access that became customary along the same corridors was actually not, so attempts to close down air access would have been illegal under international law in ways that the closing off of land-based access were not.

The blockade lasted for 15 months and across a Berlin winter, and air supply of vital materials had to include solid fuel as well as food – it was touch and go for quite a bit of the time as to whether the air bridge could be maintained effectively. About 4.5 million tonnes of supplies were shipped in by air.

I do have sources on all this, not least Barry Turner's The Berlin Airlift (2017), which I cite because it's important to have formal back up for statements here. But in the spirit of full disclosure I should also point out that I lived in West Berlin from 1972-75, and so have personal experience of the car journey across East Germany to take a picnic in the west, and of exploring the Grunewald.