You can play with the template property of ActiveField to do this.
OR if you want you can directly use one of these extensions:
ActiveForm in yii2-widgets. Just set the autoPlaceholder to true, this will convert all labels to inline placeholders (works for text inputs and text areas). You can also control bootstrap form layouts in here.
$form = kartik\widgets\ActiveForm::begin([
'fieldConfig' => ['autoPlaceholder'=>true]
yii2-label-inplace - sets the labels to inline that pushes up or down once you enter the field
For Yii 1.1 I had to extend the form class and add a method to render a label, error and input with container divs etc as this is not natively supported there. I’ve also extended the methods for each form field type to set aria-* for better screen reader support. For example set attributes to tell screen readers to watch the error divs for content inserted via JS. I will start to port this over to 2.0. Eg. extending ActiveForm may be a solution if there are not config options available for your desired solution. Or in 2.0 the new Field thingy might be a class that you can extend and leave ActiveForm untouched.