Cain Park Amphitheater is hands down the prettiest music venue in Cleveland. It even has a leg up on Blossom (and the sound quality is much better). The venue is huge, but it is still completely hidden in the lush Cain Park Forest. The brick side walls provide a stunning contrast to the towering trees. You won't believe it existed this whole time, hidden in plain sight.
I agree with others that the bands tend to be un-interesting. I typically only go once a year. But I do remember Fiona Apple rocking out to a sold out show. And when I do go, it's always exceedingly pleasant.