You can link to individual activity using the below URL structure, but it's not super reliable since the activity has no idea what stream it belongs to. In 6.0.0 each activity entry will be assigned its appropriate stream id so rebuilding from that is going to be significantly easier.
Code:
index.php?option=com_comprofiler&view=pluginclass&plugin=cbactivity&action=activity&id=ACTIVITY_ID
As for how you want to get that URL to the user that'd be entirely up to you. The activity_onDisplayStreamActivity trigger can be used to add new menu entries to an activity. Its available variables are as follows.
Code:
$_PLUGINS->trigger( 'activity_onDisplayStreamActivity', array( &$row, &$title, &$date, &$message, &$insert, &$footer, &$menu, $stream, $output ) ) );
So var7 would be the menu and is just an array of menu items. Adding a new link to that array will add a new menu item to activity.