My personal fancy goes toward puzzles and numbers.

I've got a copy of Primes and Programming by Peter Giblin, Cambridge University Press, 1993, which let me play with Perl and Number Theory in several small projects, which built on each other.

There are numerous puzzles and contests gone by which you can make your own attempt. To get you started:

&bull MJD's Quiz of the Week
&bull ICFPC past contests

There are many others. You should look for problems that interest you, or might be useful in your future "serious" projects.