Depending on your age.
The flats are really great especially in the summer.
The Rock and Roll Hall of Fame especially for anyone over 25.
Jacobs Field is the 2nd best place to see a baseball game. See my review of Chicago if you want to know what is in first place.
The slogan *the best location in the nation* was the catch phrase in the 60''s when Cleveland was not the best location in the nation by far. It has come a long way since the Indians have gotten so good and the attraction of the Rock and Roll Hall of Fame this is a place to put on your list to visit.
Quick Tips:
The best time to go is in the summer. The winters can be very cold.
Best Way To Get Around:
Driving and public transportation.