Noted among "10 Best: Outdoor Concert Venues you Shouldn't Miss!" The American Folklore Theatre, (aka Northern Sky Theater) is located within Peninsula State Park in Door County, WI http://www.10best.com/interests/travel-features/10best-outdoor-concert-venues-you-shouldnt-miss/
Pier 6 Pavilion in Baltimore. LOVE this place. Not only because I worked for Rams Head, but also because it is one of the few amazing amphitheaters left. One of my favorite shows that I saw here was Drake in the spring of 2010 (on Cinco de Mayo to be exact).
Humphreys Concerts by the bay: The Humphreys Concerts by the bay has been operating since 1982. It is an outdoor theater with a seating capacity of 1,400 persons. Its location is between Humphreys Half Moon Inn and Humpreys Restaurant. It’s a seasonal concert venue that operates from May to October.