Operating an aircraft carries a remote risk to land on unprepared ground. There is never a situation that it cannot land - if the engines fail, the pilot has little choice than to terminate the flight.
Some aircraft are prepared for landing outside of airports. The most obvious examples should be amphibians which can use both conventional runways and water surfaces for take-off and landing. In the first part of the last century the reliability of engines and the availability of airfields was low enough to make them and flying boats, which operated exclusively from water, a sensible choice for travel over long overwater routes. Also, it was easier to open a staging post on a lake than building a long runway in a remote location, and the short range of early airliners forced airlines to fly long routes in many relatively short hops.
BOAC-operated Boeing 314 Clipper (picture source)
Even today, ditching capacity is part of the design, and aircraft are able to float for some time if they are landed on water without structural damage. One example is an A320 which flew into a flock of birds and had to land in the Hudson river.
US Airways Flight 1549 floating on the Hudson river (source).
Regulations ensure that the emergency slides can be used as rafts and the life vests do indeed help to stay afloat for a limited time, temperatures permitting. However, a ditched aircraft will sink eventually, and without quick external help, ditching will quickly turn into a disaster.
If the ground is solid, flat and obstacle-free, even a modern airliner is able to land on unprepared ground. Here is a video of an Illyushin 62 landing in a field. Note that it was not anywhere near its maximum flight mass at this event. It was the last flight of this bird; now it sits idle as a tourist attraction in the German countryside.