Write something with fingerpicking.
This is pretty straightforward - You could write a folky chord progression on acoustic guitar. You could write an electric guitar part using fingerpicking. You could play a melody without a pick. It could be something elaborate or something simple. A five second blip recorded to a cell phone or a full blown multitrack song recorded in your home studio. Whatever it is, here are the only rules:
1. You must write something new, something that didn't exist before you read this post.
2. You must use your fingers to perform at least the initial/main guitar part.