Hello. You can get a lot of college credit for your service. Many AIT's have deals with local colleges to provide credit for your specialized training. You can also get credits from basic for things like leadership, physical fitness, first aid, and marksmanship. If you go to military language school you'll earn a boatload of credits.
Hello. Short answer, anything you want. The military offers a wide range of skills that you can take with you for employment in the "real world". Now, if you are referring to the GI Bill (monies you put into as well as branch of service puts into) and college courses, again, anything you want. Perhaps look into ROTC, pursuing college and officer training as well.
Go to college to study what you are most interested in learning. If you are currently in the military, the GI Bill may be available pay for a portion of your education. If you are interested in learning about military life take an ROTC course. ROTC is no obligation for one course, but there is high recruitment pressure to continue with the program. Try any service ROTC that is available at your college.