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.
Correct, as well as Rivers Casino just up River Road, if that tickles your fancy.
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 isn't a practical option. The CTA is $5 from ORD to Rosemont, $2.25 on the way back (ORD is the only station in the system that charges a higher fare). There is a free Pace bus (811) that will take you from the Rosemont Blue Line station to the outlets/theater and back.
Cab would be about $10.