For special occasions, I think NoMi is a great choice:
NoMI
800 N Michigan Avenue
(between Chicago Ave & Pearson St)
Chicago, IL 60611
(312) 239-4030
www.nomirestaurant.com
But if NoMi is a little too high for your prices, Roy's is another great place to go:
Roy's (312) 787-7599
720 N State St
Chicago, IL 60610
I did a party for my fiance's birthday last August and Roy's did a phenomenal job of assisting me. They helped create customized pre-fix menu options, setup a special section of the restaurant per my request.
Enjoy your special day.