To get to Nakano City Hall, walk from the JR Line North Exit ticket gate, cross the pedestrian bridge, and you'll find it next to Nakano Sunplaza, across the road.
And, on the back side of Nakano City Hall, near the intersection towards Nakano Gymnasium, there's a place to sit and take a break.

There are six stone benches installed.
They are colored blue and orange, and while they have a design, the seating is low and not very easy to sit on.
You might find yourself muttering "hmmm."
There's a bus stop nearby, but it doesn't seem to be for bus stop seating, so it's a bit unclear what it's for, but it might be good for organizing your luggage, etc.
By the way, there are similar benches on the side of Nakano City Hall, on the Sunplaza side, but they are narrow and don't feel like a good place to sit.
Anyway, why not try using it as a place to take a break around Nakano Station?

