The Openluchttheater (Open-Air Theater), in the middle of the Vondelpark, schedules free performances every year in the summer.
Particularly the performances on Sunday afternoons can be very busy.

Although the entrance is free, it is possible that a donation is asked.

Web:
http://www.openluchttheater.nl

Tickets:
Free, however, to guarantee a seat and support the theater you can make a reservation for E 5.-- per person through the website of the open air theater.