A BluRay or a DVD or a TV with a “HOLD” button that’ll freeze frame is good. A screen takes care of flattening 2D objects against the picture plane which is a bit “training wheels” but useful.
On top of that, studying movies and TV in particular exposes you to things like composition, lighting, editing, sound design and so on at the same time. If you’re drawing in order to learn to animate or create animated films, all that knowledge is dead useful. I’ve been drawing the compositions and action of “2001: A Space Odyssey” and it’s been really educational - and part of recording what’s happening in-shot has been gesture drawing.
Also, you can pause movies. You can’t pause real life in the same way, unless you’re drawing from a model.
Not to say drawing from real life isn’t also educational. Office workers are good for live drawing because they tend to stay at their desks and stay relatively still. I rip through thirty or more gesture drawings in my team’s weekly hour-long meeting because people are sitting down. I’m getting to the point where i can get a rough gesture drawing done in maybe ten seconds - although my workmates have started to notice me scribbling them down.
Cafes and diners are a good place to scribble discreetly, as long as you’re sticking with quick gesture sketches. Possibly libraries might work too.
As for public parks? Well, depends how windy and cold/hot it is. I don’t. I like the indoors too much.