Decent city park
The view from the top is nice, but it is ill-advised for young women to jog alone on the paths there. In the five years I've been here, I've heard of about three attacks on women jogging in the park alone. Generally it is poorly policed; friends of mine had their windows smashed out. (When this happened, witnesses didn't bother reporting anything to the police.)
It's a tough run or ride from the bottom to the top, but it's nice.
This is basically a city park - it's a big city park, but it's not so large that you can exactly go hiking there. I'm also not aware of any toilets there. The park also closes after sunset, which stinks because the view is great at night, especially for fireworks on the 4th of July.
I wouldn't go out of the way to see East Rock Park - don't drive in from out o town - but I do try to get visitors to New Haven up there to see the view.