So pull on some jeans, and some nice accessories, and hop on the train! You can actually walk from the train station to your hotel, and since your bag is so small, you can stop for lunch, or do some shopping...
This is all you have to pack - some toiletries, and some nightwear, and you're set!
Hang up your dress when you get to the hotel to change (although it has a bit of texture, so it won't show wrinkles much at all!), and make sure your tee shirt is neatly folded somewhere. This is how the rest of your brief break will look:
I know there are lots of people here in the Chicago area who take advantage of empty downtown hotels, and excellent restaurants, to take a quick "mini-vacance" over the weekend - do you?