How can I set "Assigned To" for an Activity to default to the logged-in user name?
Create a simple extension implementing hook_civicrm_buildForm like so:

function myextension_civicrm_buildForm($formName, &amp;$form) {
  if ($formName == 'CRM_Activity_Form_Activity' &amp;&amp; $form-&gt;getAction() == CRM_Core_Action::ADD) {
    $form-&gt;setDefaults(array('assignee_contact_id' =&gt; $form-&gt;_currentUserId));
  }
}