gistfile1.rb

require 'active_support/all'

class Blah
  def foo
    'foo!'
  end
end

module Things
  def self.included(base)
    base.class_eval do
      unless instance_methods.map(&:to_sym).include? :foo_without_things
        alias_method :foo_without_things, :foo
      end
      alias_method :foo, :foo_with_things
    end
  end

  def foo_with_things
    foo_without_things.sub /(\W?)$/, " with things\\1"
  end
end

Blah.new.foo
# => "foo!"

Blah.send :include, Things

Blah.new.foo
# => "foo with things!"

Blah.send :include, Things

Blah.new.foo

# => "foo with things!"