it all depends on whether u r a beginner in C or have a good programming knowledge of the data structures etc in C...
well if u a beginner.. then i suggest u do some projects like making some games aka KBC ones etc which help u learn about graphics in C
or if u have some good programming experiance in C then theres a lot that i can tell u...
!> if u know abt API's then make some good text editors using them
{do make use of mouse pointers not just arriow keys of keyboard }
@> how about making a program which searches the whole of your disks for specific files etc
or u cud also do some kinda database stuff as well
the options are endless....it only depends on the quality time that u can spend on the project ...