You aren’t trying to impress them, so much as trying to see if you and they would be a good fit. Be yourself. Be polite, and especially politely interested in whatever they’re investigating. If you really are interested, that will be cool. If not, at least make the pretense of being open-minded and accepting of the way they do things, even if you really think they’re nuts or as boring as watching paint dry. Express your curiosity. Word gets around about prospective grad students. You may not join their research group, but you may need to borrow their spectrometer some day.
Express your interests, and don’t be afraid to admit that you have interests outside of science. There are some brilliant people that essentially only think science, but the vast majority of these bright minds require a bit more breadth to occupy them. Bring some cash, so if you get the chance you can spring for the lattes. It is hardly likely that they will let you, but it feels good to be able to offer. If they’re going to be poor with you some day, they’d like to know that you aren’t a cheapskate, that you don’t expect others to pull your weight for you.
You will almost certainly have a graduate student for a host. Have a plan for what you’ll do in case this doesn’t take 4.5 hours. If it is in-between down time, you can say, “If you have something going in lab, I’d be happy to just tag along and watch.” If that isn’t practical, you can offer, “Well, if you have some of Professor So-and-So’s reprints, I can look those over while you get your stuff done.” Grad students nearly always have something left on their to-do list. Even if they insist on staying with you to chat, they will appreciate that you respect the value of their time.
Do not worry too much if you make some factual gaffe that you would have thought stupid even back in high school. That is pretty common, under the circumstances. Ornithology graduate students have been known to draw four-legged birds during their oral exam and still get their PhDs. It may get you some ribbing down the line, but it won’t make much difference on whether you get in. So you can relax and quit boning up on your biology notes. It’s either in there, or it’s not. If a little bit is misfiled, it is not the end of the world. Get some sleep and go in hoping to see some real science done. That’s what they all care about.
One more thing: I have known professors who grill students during this kind of thing… as in, they might send you to the dry-erase board to see what you can do with some sort of problem. Don’t let this freak you out. Relax. THIS IS NOT AN ORAL EXAM. Don’t be afraid, and don’t be afraid to think outloud. They want to see you in action, but they know you’re just a cub. You have nothing to fear by just cheerfully plunging right in.