Last year I got to "help" decorate for The ManFlesh's Halloween party and I did a few things you might be interested in. He has an actually entryway that's only 1 story and all the walls are wood panels or wood built-ins. First I replaced the light with a black light then I COATED that entry with spider webs, layers and layers and layers of spiderwebs - the kind you buy in a bag stretched really thin. Then I had some baby dolls dressed in costumes (got really lucky to find such in the goodwill bins). They were ratty and one said different things when you pressed her hands, feet, etc. that were super creepy in the context ("do you like my pretty dress" - that kind of thing). Anyway, I wrapped them in layers of web with just parts poking out as though they'd been wrapped up for spider-lunch and hung them in the webs. (I got the doll idea here on craftster) I also did the Martha Stewart spider egg sacks with a Styrofoam ball in a white stocking covered with plastic mini-spiders. Finally I put dozens of different plastic and rubber spiders in the web, including hairy tarantula types and glow in the dark ones. Finally, I hung clear upholstery thread from the door- and archway so that it felt like walking through spiderwebs.
I believe that if there are enough of any one type of thing it becomes sinister. So clump together your rats, bats, spiders, bones, etc.
I want to make something like this that I saw online the other day, too. http://www.grandinroad.com/jump.jsp?item=32678&maincatcode=null&subcatcode=null&itemID=17104&itemType=PRODUCT&path=1%2C2%2C2759%2C2761%2C2770&iProductID=17104