Except that creepers make no sounds while walking. I'd recommend zomies.
They do, if they are on a dirt or grass floor.
Crunching shuffling or rustling sounds.
You can throw down a layer of dirt then darken the place up nicely. Random mobs of every type will spawn up there. If there's grass, and your roof is made of any kind of stairs, light will get through and you will get daytime mobs as well.
You can even set a block of lightstone into the outer wall and frame it like it's a window, so there's a creepy light-in-the-attic effect at all times, that doesn't interfere with mobs spawning inside. Whenever visitors hear the groans and rustles, just say that's "Granny" and forbid them under any circumstances to open that door.
One idea a buddy had for Halloween is cut and paste a skeleton spawner cube using MCEdit into an area you plan to use as a house. Put it in the closet. There, now you have skeletons in your closet. Bonus points if you can also turn it into some sort of item farm.
Perhaps the best idea is to put a bunch of chests in there and fill them with nearly worn-out tools and bits of wool, string, paper, a book here and there, etc. like a real attic. Then scatter a few real rarities in the chests: a leaf block, a coal ore block, a redstone ore block, etc. Stuff that has no real value but is otherwise unobtainable.
My attic is so big like you could have 2 stories in the attic. I have a steel roof from a mod as my survival base's roof. I used the space efficiently to make a trading hall as the first floor. Above the villager stalls, I have hidden storage and redstone lines to pistons for my security system during raids. In between both of those corridors, I have open high ceiling in the middle of the trading hall. At the other end of the attic is my bedroom and additional storage in my modded survival world. In the highest space above the ceiling of the trading hall is even more storage.