Take only one object array of the class when receiving three arrays in PHP

I have a constructor that defines me 3 variables as arrays in the displayQuerys class . A function of this class fills in the array "activities" for later, in an html print this array.

The problem with which I have found is that when creating a new object, it returns the three arrays, when I only want one of them.

     displayQuerys Object
(
    [schedule:displayQuerys:private] => Array
        (
        )

    [activities:displayQuerys:private] => Array
        (
        )

    [instructors:displayQuerys:private] => Array
        (
        )

) 

The foreach stops working because I need only one of the arrays.

<?php
  $activity = new displayQuerys();
  $activity->showActivities(); // Rellena el array de Activities

  print_r($activity); // works fine

  foreach ($activity as $value) {  // here stops working
    echo "I'm supossed to do stuff";
  }
?>

Is there any way to create this object, specify which one to choose or at least isolate so that in this case you can catch "activities" and another time , choose instructors ?

Edit: My problem has been solved by adding a return in the method of the class displayQueries, in this way if I can apply the solution of Carlos:

if ($result->num_rows>0) {
     while($row = $result->fetch_assoc()) {
       $this->activities[]=$row;
     }
     return $this->activities;
   } else {
       echo "0 results";
     }
     $conn->close();
   }