Some (many?) programs will let you start doing rotations and projects the summer before you start your program. There are a whole heap of reasons for getting there early. Once school starts you'll be expected to be taking classes, doing research, selecting a lab/PI, and getting familiar with a new area (going from a small liberal arts college to a research university may involve a change of city in your case).
In your early years there is a lot pressure to do well in classes, and f you are coming out of undergrad there will be momentum to focus on your coursework, particularly as you get ready for quals. It is hard to get settled into a new lab and be productive, so your research suffers and it is hard to do well and impress potential advisors and be happy about your research work. If you start during the summer, you'll have some research momentum that will carry you through your classwork time. It also gives you time to learn the new area and all the little details you need for your life. Finally, many programs don't already have a PI picked out for you, so you can spend time going to any summer seminars are going on, sit in on different lab meetings (to learn about the culture of how the lab works), and generally start to learn about what is actually going on. Anything you are reading in papers or even on the lab website is about what the lab used to be doing, sometimes wildly out of date, and sometimes the projects that are already well established are precisely the ones you won't be able to work on or won't want to, because there is already an army of postdocs and grad students already working on them. You can scope out the landscape and find the interesting new research areas that are just emerging.
I wouldn't worry about starting a hobby right now. Once school starts, you can meet your no cohort of students. There will usually be some people who are into something new that you aren't into (rock climbing, wine tasting, soccer, skiing, distance biking, juggling, whatever). That's a good way to discover new hobbies and interests, and a good way to bond with your colleagues. Science is good about bringing together people from eclectic backgrounds, particularly globally/internationally, and it is good way to be exposed to different cultures, foods, ways of thinking, etc. One big transition from undergrad to grad school in science is that usually there is a big jump in the internationalized character of the grad students and postdocs you interact with. They will definitely be introducing you to new things. At the very least, your new university will have student groups, and usually there will be an activity fair (yeah, even for grad students), so you can have a chance to learn about new groups. You can even put together an intramural team (choose your sport) in your department.
A few years into school, you will have time and options to try something new. There is a big lull after you finish quals and classes (and hopefully finished TAing) while you are just doing your dissertation research, that is a sort of the long dark stretch. That's a good time that you will want to be getting out of the lab now and again so you don't go bonkers. Don't think about it being "my last out of the lab for a long time". Grad school is a long, indeterminate stretch of time. It's a marathon or endurance run not a sprint, so you will need to take breaks and vacations as you go. Once you are in the long research stretch, you will actually have some flexibility to take vacations and so forth. Most advisors are fine with you taking some away breaks now and again, as long as you are being really productive most of the time.
Good luck in grad school! It is can be really trying at times, but it is fun and worth it. The upcoming sequester is going to have big effects on how biological science is funded, good luck!