Примерно так должно работать:
hash.values.collect{|v| v[0]}
Пример:
irb(main):001:0> hash = { 1 => { 0 => 'apple', 1 => 'tree' },
2 => { 0 => 'foo', 1 => 'bar' }}
=> {1=>{0=>"apple", 1=>"tree"}, 2=>{0=>"foo", 1=>"bar"}}
irb(main):002:0> hash.values.collect{|value| value[0]}
=> ["apple", "foo"]