It sort of depends on which campus you're interviewing at, as well as if you're familiar with the area. You shouldn't have trouble finding a parking spot at either place. In EL, they will validate your parking, and there are tons of spots. In GR, you should be able to find a spot in the Secchia center's parking levels.
Either way, I would recommend planning on getting there a half hour early or so, just in case you run into some sort of traffic or other snag. If you get there with the whole half hour, you can just take a walk around and relax before heading inside. If you're interviewing in EL, your interview day (but not necessarily your actual interview; they split the group up and offer shuttles to other buildings for many of the interviews) will be in the Radiology building. There are some really ornate, peaceful gardens that wrap around the left and back of the building. I would highly recommend taking a walk through them, especially if you're feeling any kind of nerves.
A lot of people caution you against getting to your interview too early in fear of setting off an intervieweer's pet peeve. With the interview day itself, that wasn't the case, at least not with my experience. Given that you're not going in for your interview right off the bat (there is an introductory presentation first), getting there a bit early gives you time to chat with other interviewees and get comfortable before things start rolling. I went inside probably fifteen minutes early, and there were already thirteen interviewees (out of about 23 total) who had been seated and were busy eating snacks and talking.
Anyone who interviewed in GR have any input? I was more familiar with the GR campus before my actual interview day, but obviously I interviewed in EL.