Statue of Liberty is a must-see.
Staten Island Ferry - if you want to stay on the ferry and turn right back around back to Manhattan, but at least there are beautiful views from the ferry.
Circle Line Cruise
Chelsea Piers
Top of the Empire State Building on a clear day (buy tickets in advance at
www.esbnyc.com).
Central Park for people-watching and Central Park Zoo
Yankee Game (?) if they like sports or sports history
NBC Studio Tour (or at least hit the Studio Store)
Tickets to a live audience taping, if the girls are old enough (Letterman, Conan O'Brien, talk shows, or stand outside the Today Show window)
Times Square
South Street Seaport (?) haven't been there in a few years though - may have changed
Museum of Natural History
Guggenheim Musem
MoMA
Eat at: Sardi's early or after 8:00PM when the theater crowd leaves, hop over to Peter Luger's in Williamsburgh Brooklyn for the best steak in NYC, Mulberry Street in Little Italy for good Italian food, ColdStone Creamery for to-die-for ice-cream creations
Shopping: in Times Square, midtown on 5th Ave, SoHo on Broadway
Ah, NYC, so much to do and see.... Have fun.