I guess it depends on what you're interested in. I would head downtown, grab some dinner to go, and hang out in Voinavich Park. You'll be able to see aircraft operating out of Burke Lakefront, boats on the lake, the Rock and Roll Hall of Fame, the Science Center, and the Browns Stadium. If you're interested the Rock Hall is a great place to spend n evening walking through.
If that doesn't strike your fancy, the Art Museum in University Circle is great and is free admission. You might recognize the atrium as the filming location for SHIELD HQ in the Avengers. As mentioned above, the Crawford Auto Museum, which is in the same area, is also good.
Want a good meal? For the authentic Cleveland experience try the Great Lakes Brewery in Ohio City, or for the real Polish experience try to get to Sokolowski's. Or try a huge Corned Beef sandwich from Slyman's. Lots of good restaurants on E.4th too, although maybe a bit less local flavor.
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