I'm not exactly sure what you refer to as casual so I'm going to give you a couple of different things. But first : dress warm.. it is getting chilly here

STUFF TO DO THAT ARE FREE:

This Saturday Oct 31, 2009 : New York City Halloween Parade in West Village
www.halloween-nyc.com/

Central Park :
You can always take a night stroll in Central Park... I like the entrance near Columbus circle. It is open till 1:00AM

Strolling around,movies & restaurants
Times Square is always a good bet for strolling around since it is always jam-packed 24/7 even early in the morning. And there are really good restaurants open all night. Plus there are 2 movie theaters that screen movies till late at night.

Staten Island Ferry:
Ferry is free, last one leaves the island at 11:30PM. You can see the Statue of Liberty as you go to Staten Island
http://www.siferry.com/SIFerry_Schedules.aspx

Brooklyn Bridge:
You can walk across the bridge by taking the 6 train downtown to the last stop : City Hall/Brooklyn bridge

STUFF TO DO THAT NEED MONEY:
Dancing:
Webster Hall has 4 dance levels and it is 19 to enter. Check their website before going there since some days it may be 21+ .. entry is like $25 for guys,a bit on the high side
http://www.websterhall.com/nightclub/

Taking a bus ride:
You can take the Night Loop Double Decker bus that shows NYC at night. But it is $45 per person.. anyways here is the link to it. http://www.allnewyorktours.com/body.asp?tour=NYC-B0004&page=TourDetails

Bowling & Pool:
You can do some late night bowling & pool in Chelsea Piers... but it is 21+ Thurs to Sat
http://www.chelseapiers.com/300NYhours.htm

City sights
Empire state building : You can check out NYC at night atop the building.. it is $18 for admission and another $15 to get to the observatory open till 1:15AM
http://www.esbnyc.com/tourism/tourism_visitorinfo.cfm?CFID=35830258&CFTOKEN=89811509

Skating Rink at Rockefeller Center:
Open till midnight Fri/Sat. $14 for adults
http://www.patinagroup.com/restaurant.php?restaurants_id=74