Move to your new university and use the summer to do (at least one) research rotation.
Here's why: you said "for the next 5 years." I'm not sure where you got that time period from, but if you're doing your PhD in the US, you're going to find that it's a completely open ended process. This is *really* important to internalize, because every other form of education that you've had experience with has a fixed term: you do what they tell you to for the prescribed time period and at the end they hand you the diploma. You can't run down the clock on a PhD. You don't graduate until you can convince your advisor that you've done enough to merit the degree. And it's generally against your advisor's personal interests to let you graduate.
So, if you want to complete your degree in a reasonable period of time (e.g. 5 years), you have two tasks: 1) Find a lab with a research advisor who you like and trust, because you're putting your life in his or her hands. If you wouldn't give him/her copies of your keys and your ATM PIN, you shouldn't be in that lab. 2) Get established in that lab so you can start organizing and taking charge of your own project and working toward first-author publications.
The first step towards this is doing lab rotations. Summer is often a good time to do these, because your first year is likely to be filled up with classes which will make it difficult to spend enough time in a lab to really get a feel for it. Just make sure that the PI in whichever lab you're rotating in is going to be around (sometimes they are gone for months at a time in the summer) because the most important thing you need to get out of the rotation is deciding whether you trust the PI.
I suspect there will be several threads of people recommending various voyages of self-discovery or self-education. If you had something that you really *wanted* to do, I wouldn't try to talk you out of it, but from the way you've phrased your question it doesn't really sound like there is, and there's no point finding a new hobby this summer that you won't have time to continue once you start your program.
Best of luck with your program.