Having gone to school within walkin distance of the Bronx zoo, it was usually empty on free days as it was usually on a Wednesday if I recall. The funny thing is that none of the feral youth would ever think of settin foot in the NY Botanical Garden on free days, preferring instead to head up to Westchester or Manhattan when they cut class.
I grew up in Pelham Bay and it was a short bus ride down Pelham Pkwy. We would go down on catholic holidays (went to parochial schools). And do stupid semi legal things there. Nice place. Occasionally went across the street to the botanical gardens.
We also took the Pelham bay line down to manhattan. But we did not do the stuff the feral youth you spoke of did.
The trips, from out on the Island, With my kids in the 80s was during summer when the crowds were greatest mid week or weekend.