If you're getting into metaprogramming with Ruby, a great way to learn is by solving the metakoans.rb Ruby quiz. I assigned it as a task for a training course recently, and one person told me that it was the most fun they've had programming.
I did, however, find a small problem with the way that the koans were structured. If you solve Koan 6 by setting a class-level variable for the default value, and then implement the attribute getter so that it returns that value if it's defined, koans 7-9 pass. To avoid this particular problem, I changed the use of the number 42 in koans 7-9 each to be a different number. This will ensure that the actions of one test do not fool the assertions of another.