I'd personally go for Chicago.

I'm based in CLE right now, and fairly frequently take advantage of the $79 r/t offers to spend a day in Chicago. I fly in on a 930-10am flight, arrive around 1030am, am in the city by 11am, have a wonderful brunch, go to a museum or gallery that interests me, have a drink in one of Chicago's great hotel bars around 3pm, shop for an hour or two down Michigan avenue or simply relax with a book, have an early dinner at 530pm, and catch a cab around 630pm to make the 840pm flight back to CLE.

Granted, this is a bit more than a five hour layover, but it's still quite possible to enjoy a wonderful museum or restaurant--it all depends on what your tastes are and what you're looking to do with your time.