Rakefile

dirs = ["output/test/deepdir", "testdir", "another"]

task :default => dirs

dirs.each do |dir|
  directory dir
end

# fjean@Prometheus ~/dev/gist-806422
# master* $ rake -t -v                                                                [13:22:23]
# (in /Users/fjean/dev/gist-806422)
# ** Invoke default (first_time)
# ** Invoke output/test/deepdir (first_time)
# ** Execute output/test/deepdir
# mkdir -p output/test/deepdir
# ** Invoke testdir (first_time)
# ** Execute testdir
# mkdir -p testdir
# ** Invoke another (first_time)
# ** Execute another
# mkdir -p another
# ** Execute default

The dirs are only created as needed. A second run yields:

fjean@Prometheus ~/dev/gist-806422
master $ rake -t -v                                                                 [13:23:12]
(in /Users/fjean/dev/gist-806422)
** Invoke default (first_time)
** Invoke output/test/deepdir (first_time, not_needed)
** Invoke testdir (first_time, not_needed)
** Invoke another (first_time, not_needed)
** Execute default