Nothing wrong with visiting, and if you just want to take a tour you don't need to bug a professor to get a tour of campus (go to the Frist campus center and they have something called "orange key" tours). If you want to meet with a physics prof, make sure you email ahead of time and sell your case without overdoing it and see if you luck out. Profs are always busy, Princeton's profs are certainly no exception, but some profs are very nurturing people and love to interact with young talent (and some are definitely not). Throw some spaghetti and see what sticks to the wall. If you've got an extensive research background and solid credentials, you might even get noticed this early in the game, but this is definitely a place where talent is certainly not hard to find. And if you don't luck out with any profs, it's still worth checking out the campus. First couple times I saw the buildings and trees here i was awestruck, coming from a big city in the desert that is barely adolescent compared to this historical place.