Hertford orchestra is mostly Hertford members at the moment I believe, possibly some people from other colleges too. There's usually a performance once a term.
You have to try a pango. It's basically 1/3 pint spirits, 2/3 pint mixer. Popular in the 1st year!
Open days are usually early July, and I'd definitely go to one if I were you. Try and go to your subject open day as well. More information in the uni prospectus, which you should have/get.
With regards to visiting the college at other times, you're quite welcome to just walk into the lodge and ask if you can look around, but if you do this you'll only be able to look around the public areas e.g. one quad. I'd advise ringing up beforehand and ask if there's anyone who could show you around. That way, you'll get to see more, and get to talk to a real live Hertford member!
If you want to apply for physics (or even if you don't, to be honest) then I would be more than happy to show you around.