I also enjoy Fire Island (there are multiple sections, including family oriented), but it may be a bit much with an 11 month old with trains/shuttles/boats. As for other nice areas in NYC metro:
1) Greenport, NY on the North Fork. Beautiful, nice wineries. You can stay at the retro Greenporter hotel and get great cuisine in town. You can also take a car ferry from Orient Point to New London, Connecticut and drive back down to NYC after tooling around Connecticut/ West Chester. Do lunch in Old Greenwich, CT which is charming or go to Foxwoods or Mohean Sun casinos.
2) Monatauk, further out on Long Island is also a beauitful Long Island beach town, with lots to do.
3) NY Finger Lakes region http://www.fingerlakes.org/ for wineries/relaxation/museums/shopping
4) If you wife likes to shop, Woodbury Commons is a famous premium outlets (Dior, Gucci, Prada, etc) http://www.premiumoutlets.com/outlets/outlet.asp?id=7
5) Boston is only a 3-4 hour drive (if you go off-peak)

Enjoy your trip!