And I don't mean at home in your house or apartment, reading books in your field.
I mean ... out of your discipline, away from the university, away from other academic folks.
I genuinely believe that finding things to do outside academia is critical for anyone in grad school - even folks who are convinced that they love their job and want nothing more than to become a professor. Grad students are prone to depression and anxiety and feelings of isolation. Knowing people outside your field, with whom you can forget about the pressures of publishing and teaching, can help alleviate that stress by giving you something else to think about.
Volunteer. Join a group/club (and not just your discipline's professional organization or the grad student organization at your school).
Better yet, find a part-time job to help you alleviate the financial stress of grad school, as well as to gain some "real world experience" just in case you find yourself disillusioned with academia later on or just having trouble finding a job after graduation. If you have some office skills or customer service experience, at least you know that you will be able to find something to pay your bills while you look for your career job.
If your department frowns on working part-time like mine did? Who cares. You are allowed to do things to make you happy and keep you financially and emotionally secure. They pay you (poorly) to work for them. They aren't in charge of what you can and can't do when you aren't working for them.
And in a time when the academic job market is as tight as it is now, it cannot hurt to have some contacts and skills outside academia. If you're facing unemployment and won't be able to pay your rent, what is better - knowing that you've been absolutely loyal to your field and discipline, or knowing that the friend you made in your knitting circle will hire you as a receptionist so that you can buy groceries while you consider your next option?
You may think that you won't need these backup plans or breaks from academia, but isn't it better to be safe (and sane) than sorry?