Though everything is kinda dead I always enjoyed walking along the High-line park during my lunch breaks towards the meat packing district. (Famous people spotting area)
https://en.wikipedia.org/wiki/High_Line_(New_York_City)
You can have an affordable quality seafood lunch at Chelsea Market indoor Mall nearby:
http://www.chelseamarket.com/
^^**Google HQ across the street [111 8th Ave] caters from the seafood market in there. I know this because I worked in the building years ago.
The new Whitney museum is also actually fucking awesome. Bonus, it's at the beginning of the High-Line Park close to Chelsea Market.
http://whitney.org/About/NewBuilding
Walk over the Brooklyn Bridge or take the little East River Ferry from Manhattan to Brooklyn. Go to the Top of the new World Trade Center building.
Ride the Staten Island Ferry, shit's free round-trip. Good downtown views.
Hop on a Citi-Bike and ride your fat ass around NYC. Tons of bike lanes and coastal dedicated protected park bike lanes.