Whenever you play a game, think about the Four E's:
Environment: Check the environment to make sure there aren't any hazards. Look for poison ivy, dead branches overhead, slippery grass or dirt, and bees, snakes, or other animals.
Element: Inspect the element you are going to use. Look for rusted cables, frayed cables, and slippery or broken boards.
Equipment: Make sure that the equipment you are using is safe and in good condition.
Emotion: Ensure that your group has enough trust to succeed at the game; make sure no one will be emotionally stressed by playing the game (be wary of fear of heights, weight issues, issues with physical contact, etc)