If they already have some experience with high level languages, I would introduce them to K&R's C Programming Language. It does a good job of introducing some low-level topics, like pointers and memory management.

I also second the books that teach using Scheme, like SICP. I think learning Lisp and C introduces many important concepts.

Also, learning a web stack is very educational. Learning how a web application works (Internet protocols, server side code, databases, etc) taught me a lot.