While I get that you want a simplified example, you might want to use an example that has a slightly modified change in order to display the change.
For example:
``` ruby
def do_something(&block)
  block.call("Hello") if block
end

do_something { |text| puts text }
```

Not only does this show a modified output to show that there was a change, it also shows off another feature.
Variable passing however might be a little complex, but you could even simply change what the block says perhaps? Instead of "hello!" it could be "Greetings fellow travelers!"