I found this story starter in Beth Baruch Joselow's book of creative writing prompts, Writing Without the Muse, but it just as easily could have come from Woody Allen's Alice.
"Okay, you're invisible," Joselow writes. What could you see while invisible that you would never witness otherwise? It could be something related to your personal life, as it is for the character of Alice, who is freed from a loveless marriage when she discovers that her husband is having an affair, or it could be something completely removed from your daily reality. Maybe you'd sneak into the Metropolitan Museum of Art after hours, or spend a day messing with the mind of your least favorite politician.
Whatever it is, write it down. And if you can't make up your mind, write down all of your ideas.
Find more story starters and writing prompts here.