Dear Amy,
Both the ideas suggested by @Mark Johnson are quite good. You can also go with creating a local plugin and declaring a callback for event '\core\event\course_viewed' in db/events.php
like-
$observers = array(
array(
'eventname' => '\core\event\course_viewed',
'callback' => 'local_myplugin_observer::course_viewed_custom' );
https://docs.moodle.org/dev/Event_2
And, add settings.php for your plugin where you can add courses(course ids separated by commas) so that you can filter later the courses to be processed through event api.
Thanks.