i guarantee there must be a cleaner way, but you can access your Models that have been added as to AA by accessing the resources collection in the namespace you have defined. For example, if you have AA defined in the :admin namespace
> ActiveAdmin.application.namespace(:admin).resources.select {|r| r.respond_to? :resource_class_name}.map(&:resource_class_name)
will produce and array of class names..
=> ["::Activity", "::ActivityType", "::AdminUser"]
that gives you a list of items you can massage into a dropdown.