Besides at home, Starbucks, Metropolitan Museum...
1. Food Court at LaGuardia Airport
2. Brooklyn Green-Wood Cemetery (8am-7pm)
3. There are some serene places in Central Park, but not easy to find I would bring insect repellant if it is done outdoors at night.
Hope I could find more. The objective is to find a place of serenity, no disturbance, in one of the most populated cities in the world.