Hellobee Boards

It's gonna be hot hot hot here this weekend! If DH is still game for outdoor stuff, the Central Park Zoo isn't a long subway ride - he could try that. I'd suggest wearing DD in a carrier in spite of the heat, because there are lots of stairs/hills, and he'll have to lift her to see into the exhibits at the main zoo. More than anything, the Tisch Children's Zoo is very cute for little ones.

If you head to any of the parks, plan to get wet - they'll start turning on sprinklers this weekend and kids with water balloons/water guns have been running rampant this week (though I'm guessing your 10 month old doesn't walk, so parks probably aren't at the top of your list).

Indoors, there's the Children's Museum of Manhattan, which will have some areas suitable for toddlers.

Maybe someone in that 'hood can suggest some indoor open plays on the UES or UWS? New York Kids Club locations should all be doing drop-in plays (ie., you don't need to have a membership) and family events for the holiday, but you probably need to pre-register online. The local libraries usually have story hours on the weekend, but I believe they'll all be closed Saturday-Monday.

My LO loved Central Park, Central Park Zoo, and Brooklyn Bridge Park/Jane's Carousel.

Central Park / CP Zoo / Children's Zoo. He can buy the ticket on his phone now and just show it when he gets there. The line to buy tickets sucks.
There is also a playground near the edge of the park (5th Ave) and the 60's. There's a sandbox and swings, both appropriate for 10 months. But yeah, bigger kids can ruin it for the littlest.

If the heat is too much I would also second the Children's museum. I'm sure there's something there that would interest her.

And I think bring stroller and keep carrier underneath. Agree that the carrier is better for the zoo and maybe the museum so she can see things, But...I can't imagine wearing a 10 month old for hours in 90 degree weather while walking! That sounds pretty difficult! Plus because it's so hot Id want to carry water, sunscreen, hat, etc and that would be hard to do if you were only baby wearing (IMO). And stroller can offer some shade and place to nap. So I would want both options.

Also, since I walked around CP yesterday in the 90 degree weather - make sure he has lots of cash! I ventured out with just my CC for whatever reason (I didn't want to carry my whole wallet) And it was so hot that I wanted to buy water from the vendors in CP but they only take cash. The carousel is cash only. There are cute little trumpet players and stuff that I like to give a dollar to. Ice cream vendors. Etc. it's just good to have lots of smaller bills.

Oh and just remembered there's a "Mini" smorgasburg in the South street seaport. (Southern tip of Manhattan). It's a cute historic area. There is a big mall there in case they need to go inside to cool off. Nice views of the Brooklyn Bridge. Might be some other kid stuff but haven't been there in a while.

warn him that not many subway stations have elevators! A few do, but not many. Times sq, for instance, only has one for the A, C, E and thats in the port authority bus terminal.
So I'd have him take a lightweight umbrella stroller and stash a carrier in the bottom for flexibility.
If he's near central park, there is the museum of natural history on the west side, or if he wants a bite to eat/air conditioning, theres the food court in the basement of the plaza hotel (its a NICE food court!)
bryant park is right by times square, nice to lay a blanket out on (if they have the grass open now). A couple of decent food options in the park itself. Also a public bathroom.
Union square greenmarket on saturday morning is really fun to walk around. I like to get there early

@Littlebit7: good recommendations! I forgot to mention the subway elevator situation. I just got a Lillebaby so I can carry my toddler on my back and hopefully increase our subway usage.