What do I do with my free time now that I have graduated and got a job?

Just because you have a career now, you cannot slack off on studying.  You do need to spend time keeping up with developments in your field/industry.  You also need professional networking - join professional associations, get to know people, get involved.

You also need some sort of physical activity or sports, especially if you're desk-bound and/or in a high-stress job.  Take up sports you couldn't afford or didn't have time for.  Join a ski club, take up scuba diving, get a little boat and start learning to sail, etc - all are good ways to meet people of shared interests, make friends, become part of your community.  

Volunteer work is likewise a good way to get involved in the community, and some companies encourage volunteering.  E.g. a company I know has a tutoring program helping kids do better in school, and employees who volunteer get an hour off 3x/wk to go to the cafeteria and work with their assigned kid.

Launch a hobby - woodworking, photography, something that interests you.  Or resume a hobby you loved but had to give up/set aside for the demands of school.

Warning!  Do not become a couch potato!  It's very easy to kick back.  Unfortunately, it shows most obviously in your waistline, but bosses quickly notice that you become mentally slow, fail to keep up with news & industry developments, and fail to make connections for your next promotion or job.  And you absolutely, positively will need and/or want a promotion or another job at some point.

you should try to benefit others from the knowledge you have gained. maybe help struggling college students with their courses that you have mastered. you can always give back to society in many ways !