DUMBO, Williamsburg and Flatbush are good for a change of pace / scenery.
NYC is bigger than you might realise so you may find with what you have planned already and the more obvious sights, you don’t make it off Manhattan.
The high line and the vessel are worth an explore, best thing I can recommend is go with an itinerary and plan / map your trip maybe even with an idea of what are definite ‘wants’ to see and other stuff that is just nice to see. Last thing you want to be doing is going upper Manhattan / Central Park then going down to the WTC or for a ferry to Ellis Island, you may find yourself spending more time on the Subways than taking in the city.
Probably my favourite city in the world.