Being active makes me feel healthier and relieves stress, which in turn helps me focus on research when I need to. Here are my suggestions for how to stay active during grad school (or when you are really busy):
Bike to campus. This has been difficult with the winter weather. I have been taking the bus and I am anxiously awaiting the first signs of spring. The weather is nice enough this week (mid 40's) to bike, but unfortunately my bike is getting a tune up and may need some repairs (yay warranty). Biking is faster than taking the bus and I can leave whenever I want. (Try biking to work if you aren't in school!)
Maintain a paid membership at a gym. We just renewed our gym membership for another year. It was a hard decision because their are 3 or 4 free gyms on campus, which would save a lot of money over the course of the year. The problem is I doubt that I would ever go and we couldn't go together on the weekends. I like the routine I have at my gym, I like where it is on the square, and most importantly, paying for it motivates me to go! I know I am paying about $1.50 per day for my membership, so I keep that in mind and when I don't feel like going I think of that $1.50 going down the drain.
Stand up while you read. We have sooo much reading and research to do during graduate school that I find myself sitting for most of the day. It is also hard to stay awake during some of the more boring chapters, which can lead to unnecessary naps or eating for energy. One solution is to stand up! I will put my book or computer on the counter and work from there, or take a print out of a paper and read it while I do cardio at the gym. I will be working on this goal more this semester as the reading seems to have doubled.
Walk to bars and restaurants. In my class and group of friends we seem to find plenty of time to socialize. Walking to these places means a little extra exercise. I realize this is less possible for a lot of places in the U.S., which is why I love living in the city. There are so many bars and restaurants within a 2 mile radius (which I consider walkable, depending on the weather). They key here is convincing your significant other to walk with you.