Many people leave the back of their business card entirely blank so the people they hand them to can make useful notes to remember them by.
Alternatively, there are a number of ways to use this back space.
Some examples are:
Just your logo, large and strong.
A call to action or your brand tagline.
If you have two related businesses, you could put the individual branding for each one on different sides.
Core contact details on one side, social media details on the other.