Start with about 300 or 400 words — fiction or essay, it doesn't matter. Rewrite it using the most multisyllabic, sesquipedalian verbiage you can muster. Stretch both the language and yourself.
Ulterior motiveThough I would never suggest you approach a real writing project in this way, this exercise is no mere fandangle. This is an opportunity for you to get to know your mother tongue better.
Too many people spend their entire lives splashing around in a vocabulary the size of a kiddie pool. There are whole oceans out there for you to dive into. Go swimming.
If you don't learn a new word or two while you do this, you're doing it wrong.