One of my dorm room mates did summer quarter & sub-let an apartment before moving into the dorm for fall quarter. I wasn't there when she moved in, but she apparently had some leftover food that moved into the dorm with her. Most of the food was consumed in a few days or thrown out, but a half-bag of potatoes got shoved on the shelf of her closet and then other things got thrown up there. The potatoes were hidden and forgotten.
A month later, the room started to get an odd smell. We both claimed it wasn't us. Laundry was done, some cleaning occurred, still the weird smell. Several more weeks went by. It was 9 or 10 at night & we were both studying, door & window open, heater on. (It was cold out, but keeping the room closed concentrated the odor, which was not pleasant. It smelled like something died in the walls.) My room mate suddenly whispered, "Potatoes," and lunged across the room to start throwing things out of her closet. We found the rotting potatoes, which when uncovered were so rank we both almost got sick. It was disposed of in the utility room down the hall & my room mate spent the rest of the evening bleaching her closet shelf and its contents. (The entire hallway stank the next day until the trash was disposed of, and the utility room was difficult to breathe in.)
Not something I necessarily recommend to creep out your room mate, but it worked on me!
A couple other things you can do, which I know have worked on friends of mine:
1. Come back from the 1st day of class, start crying, scream, "I can't take it any more!" and start packing. Later that day, unpack while your room mate isn't in, and act like nothing happened.
2. When it's hot out, wear sweaters, pants, & ear muffs & complain about how cold it is. When it's cold out, wear shorts, tank top, & flip flops & complain about how hot it is.