I use the Blue Line from the O'Hare area frequently. I don't go from ORD, but use the park & ride at the River Rd stop (2 away from ORD). I agree with the other advice here, if everything runs on time, you can get an hour or two in downtown ... tops.

I don't know what the OP would want to see or do. A lot of the good stuff in Chicago isn't really near the Blue Line. So 5 hours is a little too short.

If the OP wants to get out of the airport for a while and wants something classically Chicago to eat, I would go for either a hot dog or pizza.

There is a really great burger and hot dog place in Des Plaines, Paradise Pup. It is only open to 5:00 PM. Just a little place with limited indoor seating.

http://www.yelp.com/biz/paradise-pup-des-plaines

For pizza, I would go to Lou Malnati's in Elk Grove Village.

http://www.yelp.com/biz/lou-malnatis...-grove-village

Either place is a 15 - 20 minute cab ride from ORD. There are good suburban cab companies that will get you around the area for a competitive fixed price.