$("#new_tour").on("submit", function(event){
     event.preventDefault();
     var inputs = $.map($(".sight_field"), function(input_element){
        return input_element.value;
     });
     var inputs_joined = inputs.join("|");
     $(":hidden").val(inputs_joined);
});

<div class="input_fields_wrap">
   <button class="add_field_button">Add More Sights</button>
      <%= f.label :sights %>
    <input type="text" class="sight_field">
</div>
<div>
<%= f.hidden_field :sights %>
</div>