Without having to call each session variable by name, is there a way to display the content of all the session variables currently set?
Not a simple way, no.
Let's say that by "active" you mean "hasn't passed the maximum lifetime" and hasn't been explicitly destroyed and that you're using the default session handler.
If you really need this, you must implement some sort of custom session handler. See session_set_save_handler.
Take also in consideration that you'll have no feedback if the user just closes the browser or moves away from your site without explciitly logging out. Depending on much inactivity you consider the threshold to deem a session "inactive", the number of false positives you'll get may be very high.