If I'm not mistaken, Hibiya Park used to be part of the Imperial Palace, but they only kept one part of it intact. This section is right at the park's main entrance. It's basically a small chunk of the(?) moat, which now resembles a very small lake or a pond, with little islands here and there with some interesting-looking trees. The wall of the moat blocks out the streets that head towards Ginza giving a slight break from the busy city life of Tokyo. That is, until you look higher up and see the high rises.
On the other side of the lake are many benches, 2 levels in fact, where you can enjoy this beautiful view. I would not suggest going any further. The rest is pretty barren, unless there is a function or display going on, since there's a wide field of grass for these purposes. There's also a pretty big fountain in the middle which is kind of nice.
I thought it was a very nice place to visit but it's a little out of the way unless you happen to be shopping in the ritzy area. Directly across from this park are a few up-scale hotels.