As an instructor, how to use the first day of class?
Students basically want to know if they should take your class.  To that end I would include


A short (10-15 minute) sales pitch explaining what exactly your class is about and why your topic is interesting.
Administrative details of the class (I would cover this after the sales pitch, so students who are late don't miss anything).
A presentation on the first topic in your syllabus.  This is important because hearing your first lecture will give them a good idea of the difficulty of the class, which prerequisites are required, and the style/quality of your lecturing. Also, if your entire first lecture is a sales pitch, students will feel like they wasted time coming to your class, or that your class is "easy"/not serious.