I am traveling from San Francisco to Copenhagen, I have a layover in Chicago for 10 hours. I arrive in Chicago 06:00 am and my next flight is 4 pm London.
Can I leave the airport and will I have enough time to see anything?
Can you leave the airport? Yes, absolutely, there's nothing stopping you from doing so.
Should you? With 10 hours, it's feasible provided any luggage is either checked through or you can secure it somehow.
Is 10 hours enough to see anything. Sure, you have time to see some things but not necessarily do anything like the museums.
You can take the Blue Line from ORD to downtown. CTA It takes ~45 min.
Warning: 4pm is heading into rush hour leaving the city so plan for extra, extra time getting back to ORD.