Multiple Select taxonomy as menu dropdown?

Hey,

I'm curious as to if I can force the taxonomy widget to be a dropdown menu as opposed to a scrolling box of allowed values if the taxonomy field is set to be multiple-select? The reason for this is, if I'm putting a form in a block, it's alot less space to have the taxonomy selection be a dropdown menu. On other node types I have taxonomy super select which provides the terms as checkboxes. I tried theming the form (I have overridden form template files in template.php) by saying something along the lines of:

$form['field_activities_0']['#type'] = 'select';

but when I do that no options show up. I've tried creating an array, but I don't know what to put in for default values...the $tid?

Please let me know if this is possible.

Thanks!