"Everything is an object" was one of the first things I learned about Ruby, but in Peter Cooper's Beginning Ruby: From Novice to Professional book, it is mentioned that "almost everything in Ruby is an object".
Can you give me some examples of things that are not objects in Ruby?
general language constructs, etc...
I think pretty much everything else (including methods) are objects.
After splitting the script into meaningful tokens by the lexer, everything is an object. Including classes. Even literal constants like 1 are objects. Some objects may have a syntax that is not purely OO (i.e. syntactic sugar) but that's mostly for easy manipulation more than anything. Blocks are not strictly objects though (but can as someone said be converted into one).
The most obvious one that jumps into my head would be blocks. Blocks can be easily reified to a Proc object, either by using the &block parameter form in a parameter list or by using lambda, proc, Proc.new or (in Ruby 1.9) the "stabby lambda" syntax. But on its own, they aren't objects.
Another example are operators.