The grand hall is remarkable. The vastness of it is simply remarkable and yet the building is a lively, massively busy functioning transit hub. It would have been fun to visit back in the great age of train travel when the ticket windows were open and the boards were clicking off the arrival and departures.
There is also a nice food court type area in the lower area under the grand hall and some shopping area.
Also newbie tip -- this is one of only 2 placed the subways go east and west others go uptown (bigger street numbers or downtown (smaller street number) and don't go East/West. We used the S line to get over time time square after riding up from downtown on the east side. This was not very well noted on the subway maps or google maps and had to ask for help to figure out how that S line worked.