Meg - That might be fun, too, but it might be easier for people from LI and NJ if they don't have to shlep into the city and then back out again (ok, here I go assuming everyone uses public transportation!). Just my 2 cents.

Some random thoughts:

I do like the idea of picnic/activity kind of thing. Central Park, Prospect Park, or what have you? Randall's/Wards Island Park is another possible spot, but it's harder to get to (bus). I like MyJourney's suggestion of Chelsea Piers, too.

We could meet for lunch at a restaurant and go to the park afterwards, and people could do either or both, depending on their availability/inclination/budget.

Cultural/play: The Brooklyn Museum just had a complete renovation of its entrance, the Brooklyn Botanic Garden is right next door, and both are adjacent to Prospect Park. If people want to stay in Manhattan, both the Museum of Natural History and the Met are adjacent to Central Park.