I'm in the planning stages of a "haunted attic" room where ideally I'd like to use projection ghosts and one active prop. The rest of the room I want as you described-- creepy without being overtly Halloweenish. I've been kicking ideas around and collecting things for a while now. Of course you're right that we each have our own ideas on what's creepy but some of my thoughts are--
Victorian stuff is often inherently creepy. Victorian era pictures in ornate frames, the now-long-dead subject staring out solemnly...those strike me as creepy.
I have my daughter watching Craig's List in her area for an old-looking baby buggy. Some have excellent creepy factor just the way they are.
Dolls. I don't want horror-altered dolls for my space-- just naturally creepy ones. Cracks, missing eyes, and being made with teeth all enhance the look IMO.
Dressmakers' dummies-- I might add a couple reddish-brown stains, that remind one of old, old blood (but not over the top, just a hint-- a suggestion). If I find a really cheap one already in rough shape, I'll stick a pair of scissors in it. "Egghead" mannequins (with no faces) are creepy too, especially en masse.
Ventroliquists' dummies-- I find these so freaking creepy that I'm not sure I'd want one in the house! lol!
Clown dolls, as you already have. It wouldn't fit my theme, but an empty clown costume (with mask) that looks hastily tossed aside would have high creepy factor for me.
Some of the old cast iron banks strike me as creepy. The head-shaped ones seem especially so to me. I'm still fishing for one I consider affordable.
Hourglasses, and also old pendulum clocks.
Some old books. I have a couple oldies that are in German, with Gothic lettering on the covers, and I like he way they fit in.
Candelabra -- My favorites for the "attic" room aren't Halloweenified other than with a little paint and perhaps a cobweb or two.
Mirrors have a creepy factor for a lot of people. Dark, ornate frames are good. I'd like to find a large oval floor mirror with antiqued-looking mirror glass.
I'll also look for (or possibly make) something with the general look of a Victorian coat tree. With a top-hat and a cape hung on it, and a walking stick leaned against it, I hope it will give my guests mental images of Jack the Ripper or Count Dracula without being an over the top horror prop.
Those are just a few of the items I've been gathering or looking for.
I've got a separate "lab" area, but if I didn't, I'd consider medical tools and an old microscope to be worthy additions to the creepy attic.
Have fun with your space; I hope you'll share what you come up with!