I think this is fun:

def power_set(s)
  return [[]] if s==[]
  s=s.to_a
  res=[]
  ((2**s.size)...(2**(s.size+1))).each do |i|
    t=[]
    check=i.to_s(2)[1..-1]
    check=check.split("").map{|a| a=="1"}
    (0...(check.length)).each{|j|t.push(s[j]) if check[j]}
    res.push(t)
  end
  res
end

irb(main):007:0> a=[]
=> []
irb(main):008:0> 4.times{a=power_set(a)};a
=> [[], [[[], [[]]]], [[[]]], [[[]], [[], [[]]]], [[[[]]]], 
 [[[[]]], [[], [[]]]], [[[[]]], [[]]], [[[[]]], [[]], [[], 
 [[]]]], [[]], [[], [[], [[]]]], [[], [[]]], [[], [[]], [[], 
 [[]]]], [[], [[[]]]], [[], [[[]]], [[], [[]]]], [[], [[[]]], 
 [[]]], [[], [[[]]], [[]], [[], [[]]]]]

That is one nice set. If you could have any of the elements in that set, which would you choose?