Can you reference specific, proper-noun products/songs/locations/etc. in your screenplay?

By all means, yes. Being specific often helps your scene feel more grounded, or funnier.

But keep in mind there’s a difference between mentioning the name of a real thing, which is generally fair game —

REGGIE

That dude’s a top hat and monocle away from being the tycoon in Monopoly.

— and the thing itself. If you show characters playing Monopoly, you’d probably need to get the rights. If your movie featured a shoe and a car racing to get to Boardwalk, you’d be getting a nasty call from Hasbro.

In general, it’s a bad idea to hinge anything integral on copyrighted material someone else owns. If your entire plot stems from a character hearing All-4-One’s “I Swear” and reacting to specific details in its lyrics, that could be a problem. What if you can’t get the rights?

So the short answer is yes — as a writer, write whatever is most interesting. “Denny’s” is generally a better scene heading than “nondescript 24-hour chain family restaurant/diner.” Just understand that the scene you wrote in Denny’s may wind up taking place in IHOP, or a fictional Donnie’s.