I know this is not a revolutionary idea, but it's been fun and had generated some good ideas, so I thought it would be a good thing to share here:
If you have someone you know who is also a writer (for me it's my mom), or someone you think would enjoy something like this, start up an email chain where you provide a sentence with a simile or metaphor, and then an additional unfinished sentence. It's the other person's job to finish that sentence that includes a simile or metaphor and then provide you with an unfinished one. Round and round they go!
With my mother and I it has turned into an actual story idea, and I think in most cases it will because it's easier to write something that relates to the previous ones than make a brand new idea.
It's also fun because it's no pressure and open-ended, you can write down the first thing that comes to mind and then send it back. And it's also fun waiting to see what the other person sends you!
In our case we very quickly moved from single sentences to multiples and even full paragraphs, and sidebar discussions about where we thought the story would go (often realizing that we both had the same ideas).
I suppose, sort of like music writers, at a certain point you can both decide that the ideas generated are something you want to work with and you can separate and each write the story that has started to evolve, or you can continue until some sort of conclusion happens. In either case start up a new one and see where it goes!
In our exchange we started with "I poured wine into that glass like a ____", and now we have a herd of cows and an inexperienced cowhand trying to get them to Farpost without the original leader who was killed (or was he?) in an attack by Indians.
There was also an interesting turn early on where I thought we were a Roman Legion but it turns out we're cowboys!
Edit: Something else I thought of about why this might work better than the more general "I send a sentence and you send one back" is the simile / metaphor restriction. Just like in another one of my hobbies, music, it helps me to place restrictions on myself when trying to create something new. If you have all the instruments and plugins and fancy doohickeys available it can be paralyzing. But if you box yourself in a little it's sometimes easier to make something that you can expand on later. But whatever works, go for it!