Riff off of an existing "poetry machine" algorithm like cut-ups, erasure, or diastic.
Create a mad libs generator.
Create a "word replacer" (all male pronouns with female, all fruits with vegetables, etc.)
Investigate RitaJS to see what kinds of metadata you can glean from a string. Here is a quick and dirty example that shows you how to get parts of speech tags for words.
Interesting sources for text
If you find interesting sources for text input, list them here.