CiviCRM Entity and editing or viewing activites
With CiviCRM Entity, there is a Drupal form for editing activities
/civicrm-activity/[activity-id]/edit
Unfortunately the form does not limit to only those custom fields that are appropriate for the activity type.
In the past, we had done hook_form_alter() implementation in custom module to hide custom fields depending on activity type.
This feature to only show custom fields appropriate for activity type is on our radar for future improvements.