Another good spot (in Cranston) is the hill in Thornton off of Atwood Ave. You can see Providence laid out below you and watch the sun set. In fact, it is the exact opposite view of what you see from Prospect Terrace. If you look carefully when you are at Prospect Terrace, in the distance you'll see cell phone towers flashing on a hill.
That is the hill in Thornton. I've watched the sun set from both places. Pretty neat.
Do note, however, that the area in Thornton is a residential area, not a public park. That having been said, I don't think it would be an issue, as long as you weren't planning on having a picnic or anything.