Are all flights planned?
You didn't specify the jurisdiction that you were asking about, but in the US there is a regulation that covers the minimum that you must do before flying.

Much of it is common sense:


You want to check the weather, because it can impact you much more than it usually does in your car and you don't want to get stuck in a really bad situation.  
You want to check the Notices to Airmen (NOTAMs) to make sure that you can use the airport (i.e. it isn't closed, or the runway isn't closed, etc.)
You want to make sure that you have enough fuel, because you can't just pull over at the nearest gas station when you get low.  
You want to make sure that the runways are long enough and that the airplane can climb well enough so that you don't crash into something.


That being said, if you are familiar with the area and the airports in the area, you have used them before and are already familiar with the performance numbers for your airplane, then you don't really need to do much more than check the weather/NOTAMs and make sure that your airplane is legal and safe to fly.

Things are a little different than back in the barnstorming days, but then there are fewer accidents too.  :-)




  §91.103   Preflight action.
  Each pilot in command shall, before
  beginning a flight, become familiar with all available information
  concerning that flight. This information must include—
  
  (a) For a flight under IFR or a flight not in the vicinity of an
  airport, weather reports and forecasts, fuel requirements,
  alternatives available if the planned flight cannot be completed, and
  any known traffic delays of which the pilot in command has been
  advised by ATC;
  
  (b) For any flight, runway lengths at airports of intended use, and
  the following takeoff and landing distance information:
  
  (1) For civil aircraft for which an approved Airplane or Rotorcraft
  Flight Manual containing takeoff and landing distance data is
  required, the takeoff and landing distance data contained therein; and
  
  (2) For civil aircraft other than those specified in paragraph (b)(1)
  of this section, other reliable information appropriate to the
  aircraft, relating to aircraft performance under expected values of
  airport elevation and runway slope, aircraft gross weight, and wind
  and temperature.