Does anyone have suggestions on the best things to do with a family on a hot and humid weekend in Manhattan?

Yeah, stay inside. Shopping, museums, etc.

Department stores, museums, wear sundresses, and drink lots of water.

Circle Line
Staten Island Ferry
Hop on/off bus tours (stay inside the bus where there's a/c)

All those other ideas and stay out of the subway!! the heat is the reason I won't go to NYC in the summer!!

Actually the subways are much better than they used to be. The platforms are brutal (no a/c, just a fan here and there) but most of the trains do have a/c.

Regarding the subways, take a sweater. I couldn't believe how freezing cold so many cars were -- and they seemed even more so after waiting on the sweltering platforms.

Don't forget movies -- especially IMAX or somewhere like the Ziefield, depending on what's playing.

GoTravel made one suggestion though I never do -- wear sundresses.

Get a lot of your outdoor walking done early in the day. Schedule indoor visits for when it's the hottest. Bring a reusable water bottle and fill it up whenever you can.

I like to rest in the hotel room from about 4-6, which is generally when it's the hottest outside.

Wear light colors and fabrics. Bring very comfortable shoes that don't rub - my feet swell a ton in New York.

Soak your feet in a cool tub when you get back from walking.

Agree to do outdoor stuff in the am or after dark. Spend afternoons either on a boat or inside someplace cool.

As for travel, subways are the pits - sometime the staitons get to 110 due to the hot air blowing out of the car. I won;t ride them between July and Sept. Either take a city bus - or if there are a couple of you and the distance isn't that great take a cab - all have good AC. (If the driver doesn;t have it on tell him to - he's required to do it by law.)

Actually I'm w/ neo - I got on a train today and I was freezing - and happy to be riding only a few stops. But the platforms can be brutal.

One of the things we do is ride either the water taxi, the beast, or the Staten Island Ferry - also the Hoboken/Jersey City ferry across the Hudson River is lovely- and not too expensive.

Stop in a cafe for an iced coffee break - there are great choices in SOHO and Little Italy.

My daughter and I walked across the Brooklyn Bridge during a heatwave and found it breezy and shady in the middle. Of course, we did have to walk back : )

Other than that, I'd suggest the ferry rides, indoor museums or shows.

Bear in mind that the tall buildings keep half the street shaded. It was nearly a hundred degrees the day we went, and we still had a great time.

How old are the kids?

There's a children's museum on the upper west side. The Museum of Natural History (book tickets in advance the lines are long). There's a fireman's museum downtown that I hear is great for boys.