Midwest Young Artists, the local youth music program, hosts chamber music, jazz, orchestra, and individual instrument workshops at Ft. Sheridan in Highwood. If you are taking summer school, they have evening ensembles!
Located in Bloomington, Indiana, the IU Jacob's School and Band Department host a one-week immersive music camp. As Ms. Holt worked at this camp for a long time, here is a more detailed description of how this camp (and many below) works: Students currently in grades 8-12 will learn, rehearse, and perform a full ensemble program with wonderful conductors. In addition to full orchestra, concert bands, and (auditioned) jazz bands, students have sectionals with instrument specific staff, a master class with Jacobs School of Music faculty, and a daily chamber ensemble or elective class such as conducting, composition, jazz improvisation, music theory, or leadership. (PS. Ms. Holt can get you a scholarship to this camp!)
University of Illinois hosts several week-long music camps in jazz, classical, and specific instruments.
Summer jazz camp as well as a drum set camp.
Located in Michigan, this is one of the oldest and most prestigious music camps. It is both intensive and competitive.
Also located in Michigan, this is also one of the oldest camps. Great for students who want a more traditional "camp experience" with their music.
Located in Door County, Wisconsin, Birch Creek hosts two-week orchestra, jazz, and percussion camps.
This is one of the oldest and best improv camps in the nation, hosted at the University of Louisville.
This local college offers a Jazz Combo Camp and a Flute Camp.
Eastern Illinois University hosts a camp for drum majors.
North Central College in Naperville hosts another good camp for drum majors.
Many other colleges sponsor summer music camps. A quick search with the name of the school and "summer music camp" will get you to the right site.