I think no matter where your at people will be having a good time..some places do private rooms for free...golden corral does this, also famous daves lets you have the back room off 120th and L for free, many churches will let you rent out a room with a kitchen for pretty cheap and you could do a potluck. Just depends on what your looking for..someone told me some pizza place has a room you can get for free too..in elkhorn I think. I cant remember..lol GL! I too need to figure out a place close to my wedding location.
With 18 people you could just about have it anywhere if you called ahead and reserved it. So, you could go to Old Chicago and have pizza, or Spaghetti Works, or even a steak place. What about a place like Dave and Buster's? They would be fun and you could hang out with everyone for more than the meal. I know they have party rooms that close off to the rest of the restaurant. Good luck.