Is there a way to create methods just for the instance of a Ruby class from inside that instance?

You need to grab a reference to the instance's singleton class, the class that holds all the instance specific stuff, and define the method on it. In ruby 1.8, it looks a little messy. (if you find a cleaner solution let me know!)

Ruby 1.8

class Example
  def initialize(test='hey')
    singleton = class << self; self end
    singleton.send :define_method, :say_hello, lambda { test }
  end
end

Ruby 1.9 however, provides a much easier way in.

Ruby 1.9

class Example
  def initialize(test='hey')
    define_singleton_method :say_hello, lambda { test }
  end
end