Writing a brainfuck interpreter in a language is a good way to size it up. Unlike "hello world", it actually gives you a meaningful impression of how string handling, data structures, etc. work. It's not hard, but it's just enough to work up a sweat -- like a web-programmer writing a blogging engine or a wiki to try out a web framework. (If it is hard in the language, that should also tell you something.)

See also:

* Project Euler (http://projecteuler.net/), for math problems

* Programming Praxis (http://programmingpraxis.com/)