That you can't do without PHP as you need to act on the display output of the activity, parse specifically for your activity type, then override its display. The trigger to do that is activity_onDisplayActivity. You'll have to use a Code action with Method set to PHP and under Parameters set variables 1-8 to be references. You can then modify the activity output from your action. The trigger variables are as follows.
&$row, &$title, &$date, &$message, &$insert, &$footer, &$menu, &$extras, $stream, $output
Specifically in your case you'll need to modify $title, which would be done as follows in the code action.
Code:
$vars['var2'] = 'This is my title';