What Are Your Favorite Interview Tests?

I have a number of candidates I have to interview over the next two weeks and it's been a while since I've interviewed anybody. In the past one of my favorite tests for candidates is to build a basic programming language with the candidate. It's a lot of fun and really demonstrates core knowledge.

I was wondering what other people like to do and have found successful.

[EDIT] Sorry, should have said this is for a Ruby on Rails engineer.