Summer is when I make most of my money so Im ususally to busy to plan trips then. But I would suggest blues fest in chicago, birthplace of the electric blues. Its a weekend in june and used to be free (probably still is). Tons of great (real) musicians who have been ripping it on their respective instruments for decades. And if you ever want to visit chicago, june is the start of the 4 month window where it is actually tolerable to be outside.