
<input type="checkbox" name="Interests[]" value="Guided walks" id="Interests_3" />
Then to use the implode function, change:
$MailBody = $MailBody . ((isset($_POST["Interests"]))?$_POST["Interests"]:"");
to:
$MailBody = $MailBody . ((isset($_POST["Interests"]))?implode(", ", $_POST["Interests"]):"");
the first argument in the implode function is the separator. I used ", " to create a comma separated list.

On what page is this code to change? I can't find it on my form or the template page. I am only getting the last box checked ... not all boxes checked.
Brad