I guess it depends on your role and creativity. I work on a workflow product. Our lead developer has been known to secretly log in and change the color scheme for the product on the day before a demo so that it fits with the holiday (red/white/blue before July 4th, red & green before Christmas).
One year, he turned it red and pink for Valentine's Day, and then renamed the product and some of the pages to "Love Connection." Then he proceeded to create a workflow (using our new functions) that was like an online dating site. So instead of processing print jobs, the workflow was basically a decision making engine to help someone find their "perfect match." It was a bit of work, but it was wildly entertaining.
Other times, our scrum master or other team members have actually recorded the demo in advance-- so they could do them in their best "radio announcer" voices and work up a little routine.
All of those things might be a little much to do for every demo, but changing it up on occasion can be fun.