Late Evening near the Marriott on Magnificant Mile
You have two of our best small plates places within a few blocks:
Two more lean a bit more towards the wine than the food, and are also within a few blocks:
Here are two more very lively places nearby worth considering, although they're not specifically small plates type places:
They're all open pretty late, although some of the hours vary depending on whether you're just drinking, or eating from the bar menu, or eating from the dinner menu. Nacional 27 is about a ten minute walk from the hotel; the others are all less than a five minute walk.
I definitely second the suggestion of Pops for Champagne, however I will say that in addition to great champagne and cocktails, the food I've had there has been excellent! Service is also great and atmosphere is perfect for a girls' weekend.
I have recently been to Sunda and, I agree with nsxtasy, while not specifically small plates (but definitely "shared plates"), it would fit the bill for you. Very trendy (lots of reported celeb. spottings), and all the food at my table was delicious- a top spot for Asian fusion.
Quartino is solid for Italian small plates and a glass of wine, but I wouldn't go out of my way for it (nothing terribly special there and Pops for Champagne is across the street- go there instead).
If I were you, I'd skip both Sable and Nacional 27- the craft cocktails at Sable are good, but the drinks at The Drawing Room (not too far away) are better, and the food at Sable is VERY underwhelming- I've had about 5 of their small plate items that were all extremely disappointing.
One place I'd add is Purple Pig- great location right on Michigan Ave, great small plates (vegetable dishes are as good as the meat items they are known for)- everything is great for sharing. They don't take reservations, but if you go during off hours (late lunch, early dinner, or late-night), you won't have a problem.
I mostly agree with this advice, but my opinion of the food at Sable is exactly the opposite - I've found it to be exceedingly delicious and creative, one of the very best contemporary American restaurants in the city. Chef Heather Terhune is at the top of her game. I posted a detailed report of one dinner there at http://chowhound.chow.com/topics/704524 IMHO there's no other place that does as good a job at providing great food AND great drinks. I consider the food better than at any of the other restaurants mentioned here. Try it and decide for yourself, and let us know what you think!