active_admin.rb

# It extends activeadmin to show pretty boolean values
# 
# config/initializers/active_admin.rb

module ActiveAdmin
  module Views
    class TableFor
      def bool_column(attribute)
        column(attribute){ |model| model[attribute] ? '✔'.html_safe : '✗'.html_safe }
      end
    end
    class AttributesTable
      def bool_row(attribute)
        row(attribute){ |model| model[attribute] ? '✔'.html_safe : '✗'.html_safe }
      end
    end
  end
end

# example
# app/admin/user.rb

ActiveAdmin.register User do
  index do
    column :name
    column :email
    bool_column :admin
  end
  show do
    attributes_table do
      row :name
      row :email
      bool_row :admin
    end
  end
end