classes.rb

BORING = [BasicObject, Object, Module, Exception].freeze

def classes(obj)
  obj.constants.map do |constant|
    obj.const_get(constant)
  end.grep(Class).reject do |klass|
    BORING.include?(klass) || klass < Exception
  end
end

results = classes(Object).reject do |klass|
  klass.name.include? '::'
end.to_h do |klass|
  [klass, classes(klass)]
end

pp results