I just checked the ORD area and looks like there's a Rosemont Theatre, Fashion Outlet and a couple of restaurants just outside the airport. I think that would be a good place to spend the day if I get there early.
So how easy is it to get there? I see the blue line is $5 each way between O'Hare and Rosemont, or is it possible to walk from Terminal 5? How much roughly would the taxi be?
Walking would not be advisable. It would take you a while and the area is not walker friendly from the airport. The Blue Line would get you there it would be $5 from the airport and $3 back. They increase the standard fare for all trains leaving ORD. Rosemont is not a particular exciting area in my opinion. The Fashion Outlet, however, is pretty new and nice for shopping. I don't recall any must visit restaurants in the area.