Implement (without looking) or re-implement (after looking) the examples and exercises from the following or similar books; the book's language doesn't matter:

The C Programming Language, http://en.wikipedia.org/wiki/The_C_Programming_Language

The Elements of Programming Style, http://en.wikipedia.org/wiki/The_Elements_of_Programming_Sty...

Writing Efficient Programs, http://en.wikipedia.org/wiki/Jon_Bentley

Programming Pearls, http://netlib.bell-labs.com/cm/cs/pearls/

The UNIX Programming Environment, http://en.wikipedia.org/wiki/The_UNIX_Programming_Environmen...

And, er, Learning Python, http://en.wikipedia.org/wiki/Learning_Python