Write a story while sitting on a comfortable chair/bed/couch/whatever.
Write a story while sitting on a hard/uncomfortable chair.
Write at a table.
Write on a clipboard on your lap.
Write on your computer.
Write with a flimsy book/magazine/newspaper/catalog under whatever you’re writing on (as in not on a desk or something sturdy).
Write while standing up.
Write with the paper on a wall. (Don’t bother trying to use a pen for this one.)
For extra credit, write about someone in the same position as you are (as in someone who is standing while you’re standing and writing, etc.).
How did your mood change from story to story?