In the form that is being called...ie. <<< I have two files, one that contains the HTML with the form. and "another that the form calls " and inserts all the info into the DB. So while all of this is making >>>>
$str_activities = $_POST['activities'];
$str_activities = preg_replace("#<.+?>(.+?)</.+?>#is", "\1", $str_activities);
the above code is the one that u need to change to an loop to look for the values in array of activities.
comming to the database part of it, in the database have u got one field set for the activites or more than one. because u need more than one field in the database for the activities because it gonna be more than one activities that get selected. If u want only one field to get into the database might be u need to combine all activities into one string and then insert it into the database.
"I might be wrong because i dont know how u r database is going to be and what u r gonna use the activities for".
hope that gives u an idea of what changes need to be done.