While many contests accept submissions that allow the author to select that nature of their work's contents, NYC Midnight gives the authors restrictions. This year, my round one requirements were: genre: fairy tale, setting: a barn, object: crystal ball. I was also limited to one thousand words and had forty eight hours to submit my piece. The deadline forces me to focus and the time line is just long enough for one round of edits and not long enough for me to overthink anything. I'm usually very proud of my entries for this contest.
Not every writing contest is so rigid or such a whirlwind. Many give you months to enter and only limit the word count for entries. The crazy deadlines of NYC Midnight's contest may not work for you. That doesn't mean there won't be other contests that will.
The internet is filled with resources to help authors find contests worth entering. Here are a few helpful links for those looking for a place to get started.
http://www.pw.org/grants
https://www.dystopianstories.com/writing-competitions-2016/
http://www.writersdigest.com/writers-digest-competitions
http://www.freelancewriting.com/creative-writing-contests.php
http://www.writermag.com/writing-resources/contests/
I'm sure there are many more, but this is a handy short list to get you started. I may have to take my own advice and see what other contests there are to throw myself into. SO, go forth, find contests, write, and, most importantly, have fun!