You are going to spend at least 4 years or more in graduate school, and around 3-5 years in a postdoc position. That's a big chunk of your life, so apart from doing experiments, writing papers or your thesis and maybe doing some teaching, what else are you doing?
To paraphrase one of my favorite books (So What Are You Going to Do With That?: A Guide for M.A.'s and Ph.D's Seeking Careers Outside the Academy by Susan Basalla and Maggie Debelius), what will you have to show for yourself besides teaching and research at the end of it all? For most people, the answer is "probably not much". But what Basalla, Debelius and I suggest is that it would be very beneficial for you to actively pursue other interests or hobbies while in grad school or doing a post doc.
Why you should make time for your interests and hobbies
Teaching and research involve a variety skills, including project management, organization, complex problem-solving, and analytical skills, to name just a few. But expanding your skillset by pursuing other interests or hobbies is well worth your time. Not only will it make you a more interesting person, it will also make you more marketable to both academic and non-academic employers. But the most important reason of all is that it will make you feel alive, and maintain your sanity. Science is exciting and it's your passion (or is it?), but it's also demanding so having other passions can help you keep your life balanced.
Consider new options
It's always a good thing to be open to new possibilities and options that you might never have considered before. My hobby is learning, and consequently, I am a raving fan of courses. In doing one of these courses I discovered that I love translating, and that I'm good at it (so my alternative career could be in translating scientific texts, I'd love that), and it was also through a course that I discovered my affinity for life coaching. Both of these courses began as hobbies, just for fun, and developed into something more. Just pick something that interests you and go for it — you'll have lots of fun and recharge your batteries at the same time.
The career benefits
Apart from personal satisfaction, having a hobby has added benefits to your career. New activities will broad your experience, add important skills (for example group activities will show your ability to work as a part of a team), strengthen your network and expand you options. You can also get recommendations, and it's an ideal way to learn about a new field and experience first hand if you'd enjoy or not doing something similar professionally.
This is especially important if you are considering a career outside academia as you can show potential employers that you have other interests and abilities beyond the lab. But even if you're happy inside academia, it will serve you well too.
Fitting it all in
I know you're busy and don't have much time. Or that's what you tell yourself. But how much time do you spend actually working? And how much time do you spend pretending to be working, feeling guilty because you should be working or avoiding getting to work? If you track down how you use your time (and how you waste your time) you can be more efficient and find time for other things. Check out this article on measuring your fudge factor to get some ideas on how to do this.
So what are you waiting for? Make the most out of you grad school and postdoc years. There are lots of different things you can do, from learning a new language, to developing skills like leadership or communication, or volunteering your time, taking a part time job, writing for a university publication or writing a blog…. Take advantage of your University or department programs.
And if you don't know what to do, drop me a comment, I'd love to help you find out.