Linking directly to Contact's Activity from Scheduled Reminder Email
SOLUTION FOUND
It wasn't rocket science but I'm putting it here in case others need it (or when I forget).

Here's how to create a link that will take you directly to an Activity in CiviCRM.

EDIT ACTIVITY PAGE ON WORDPRESS CMS

http://www.MyOrganization.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm/contact/view/activity&amp;atype=2&amp;action=UPDATE&amp;reset=1&amp;id={activity.activity_id}&amp;cid={contact.contact_id}&amp;{contact.checksum}&amp;context=activity

VIEW ACTIVITY

http://www.MyOrganization.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm/contact/view/activity&amp;atype=2&amp;action=VIEW&amp;reset=1&amp;id={activity.activity_id}&amp;cid={contact.contact_id}&amp;{contact.checksum}&amp;context=activity

If you're using a different CMS you can find the root URL structures at https://wiki.civicrm.org/confluence/display/CRMDOC/Tokens