Gotta gotta gotta come here on a Sunday afternoon around 1p. Around this time they have something called the "Tam Tam", this is where a bunch of strangers come to play their drums and make their own music. All you have to do is listen out for the drums and that will take you to the "Tam Tam".
The park is also filled with people doing activities such as tight roping, hackysack, music, old war fighting with plastic swords, etc...
Besides all of the activities you can take a mile and a half walk to the top of the park there you will get a great view of Downtown Montreal. Keep in mind the walk might get a bit tiresome.
Park is also a nice place to take your bike and go for a stroll as there are lot of trails to visit.