As you just have one day - there is not much preparation for you to do.
Couple of ideas:
1. Create and sell hotsheets. Could be on how to sell in your city. Or on the topic of the expo.
2. Spray print a few tee-shirts in a night and sell them in the parking lot.
http://www.instructables.com/id/E9S4...JZ8O/?ALLSTEPS
3. Just collect brochures from all the stalls - and see if you can earn some finders fees later on...