The hike along the Na Pali coast is one of the most beautiful on Earth, and one of my personal favorites. The problem is that it is not a good hike to take young kids on. The cliffs are high, the paths are slippery, and it is difficult to get medical help if you should get hurt.
That said, I did take my 6-year-old son on the hike when we were in Kauai the last time we went. We only hiked about a quarter of a mile though, before the path began to border some of the cliffs. He was fine for that bit, but I wasn't ready to go on to the first beach. I have done the first leg of the hike without my child, and it was pretty grueling because of the heat and humidity.
Go take a peek of the coast, but spend most of your time at Ke'e beach where the trailhead begins.

When I took my son to NYC for the first time this summer, I gave him the choice of visiting the Central Park Zoo or the Bronx Zoo. He chose the CPZ because it's the setting for one of his favorite cartoons, the Penguins of Madagascar.
He did get to meet the penguins and see some of the architectural landmarks that are often seen in movies. He loved it.
This is not a big zoo, with big animals, or a lot of variety. It is a good place for younger kids and anyone looking for a nice break from the big city for an hour or so. I was glad we got to go to a zoo, but didn't have to spend most of the day exploring it. There's too much else to do in NYC.

I have to admit I was more excited about the mule-drawn boat ride than my son was. But when we were done, he had to admit it was pretty cool. You get to see how the workers (all dressed in period costume) do the tricky steering through the locks and the handsome mules pull you slowly along. The tour guide on the boat was engaging and informative.
We happened to go when on a 99-degree day, and we made the last ride for that day (around 11 in the morning), because they have to cancel boat tours if the weather gets too extreme. The boat tours are seasonal, so check the website.

Because it's usually cold and foggy in San Francisco, the conservatory is the perfect place to warm up for a bit. Besides the warm, humid air, you will find that there are plenty of plants for young kids to get excited about. Venus flytraps anyone? They do sometimes have special exhibits, including a super cool model train one.

Yes, it is an art museum, so most toddlers will be rather bored. But like most attractions geared towards adults, you'll have to modify your visit (and expectations). We have been going to the De Young since my kid was a preschooler, and we get different things out of it each time.
My son's favorite part of the museum is actually totally free for everyone in the whole family: the observation deck in the North Tower. Just head past member's admission desk to the elevator up to the Tower.
It is good to note that kids under the age of 12 are free (except for some special exhibits), so you can feel less guilty about not seeing everything in the museum. The food is pricey, but so are most of the places in Golden Gate Park.