a variable to hol the category value and an if statement should do the trick:
<table width="100%" border="0" cellpadding="0" cellspacing="0">
<?php $Category = ""; ?>
<?php do { ?>
<?php if($Category != $row_ItineraryProfiles['Category']) { ?>
<tr>
<td><?php echo $row_ItineraryProfiles['Category']; ?></td>
</tr>
<?php
$Category = $row_ItineraryProfiles['Category'];
}
?>
<tr>
<td><?php echo $row_ItineraryProfiles['Activity']; ?></td>
</tr>
<?php } while ($row_ItineraryProfiles = mysql_fetch_assoc($ItineraryProfiles)); ?>
</table>