Though not River North, you might want to consider Brett's. A wonderful, warm and cozy spot, serves an excellent brunch. I haven't been there lately, but my memories of brunch there are wonderful and I wouldn't hesitate to return. It's not terribly large, though certainly large enough and not packed.
(I think she just changed the name, actually, to Brett's Cafe Americain--like Casablanca)
2001 W. Roscoe
It's on the border of being in the South Loop, on Harrison, west of Dearborn. Great, kind of funky brunch food. They have excellant pancake flights - four different kinds of silver dollar pancakes based on a common theme (like chocolate or wine), as well as interesting omelettes.