ive always found that the best way if you think you wont be able to cope with not having anything planned is to think up a number of different characters and situations independent of each other. if you do it this way, then you wont get caught out by having designed a story that doesn't fit the question and sounds like its been pre-written and you'll have lots of situations to choose from.
You dont need to focus on just one osrt of journey - do the concept of journeys in general.
oh, and about the 'needing to be original' etc. part, this is what works for me - it was too much trouble to try and think up something completely new and original in terms of storyline in the exam, so i didnt try. instead, i took a very cliched storyline - a doctor having a midlife crisis, and skewed the perspective - i told the entire story form the point of view of the rubber plant in the doctor's office. it made the story seem a very original and creaive piece, when in fact, the basic premise of the story was a veryv common one. try experimenting with something like this.
oh, and one more thing. if u can, try and practice writing in stream of consciousness - if done well, this is always looked on favourably by the markers. however, its not easy, so i wouldnt recomment trying to do it in an exam if you havent practiced before
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