You have many choices, Systems administration (controlling the behavior of your operating system & machine), network administration (how you machine interacts with the world), software development, including applications development (programs that run on unix, etc, so you would get an opportunity to expose yourself) and OS development (actual development of the unix operating system). There are many other areas.

killerserv gave you a good set of links above, in each of the suggested areas, you will get an opportunity to use Unix in one way or another.

IMHO, the another way to also learn Unix, is to learn the C language in a unix environment and use the C/Unix libraries, writing software programs, as a developer (if that is something you would like to consider?), it will give you a different perspective of unix.