You can get to Atlantic City for very cheap. The Academy Busses which leave from the Port Authority cost about $25-$30(RT) and then, depending upon which casino they bring you to (it differs based upon time of departure) you get $xx in cash back and/or $xx towards food at the hotel.

You could also grab some take-out lunch and take the train to Hoboken (PATH train) or Brooklyn (subway)... sit by the river and eat your lunch with a postcard view of the Manhattan Skyline. In Brooklyn i'd head towards "the promenade in Brooklyn Heights" and in Hoboken i'd head towards "Pier A" park.

Also free, is to walk the Brooklyn Bridge... you can walk it both ways, but if you're tired and don't want to walk as much, take the subway to Brooklyn and then walk the bridge back towards Manhattan... taking in the NYC Skyline with other NYers.

There is also the ferry to Staten Island which passes right by the Statue of Liberty. You can stay on the ferry and simply roundtrip it, hopefully timing it so that your return to Manhattan is near dusk/sunset and you can see the city glow from the reflection of the sun as the lights of the city begin to illuminate.