Fuzzbo: A co-worker of mine was looking for a romantic get away for he and his wife --35 year wedding anniversary. He looked Sedona/SFO/Savannah/Hilton Head..all those 'couple' type cities. He ended up going to St. Louis!...Don't sell yourself short..STL is a good place to visit with kids.
(well--maybe not today..is the electricity running yet??)

I second the idea of Portland, OR, along with a few days on the coast. Cannon Beach would be wonderful for the parents and the child can run along the sand and take in the tide pools (you're likely to have good weather through September). It would be relaxing and cool (when the temp toppped 100 in Portland we used to drive down to CB where it was usually at least 15 degrees cooler). The family can then hop in the rental car up to Seaside to take in the (age-appropriate) stuff there.
Plus, the plane ride from LA to Portland is much shorter than a plane ride to NYC. And you can get from the airport to downtown Portland in a matter of minutes, either by car or light rail.

I grew up in LA and our family vacations beginning before I was a year old were camping thru the fantastic state and national park system. There are places in the desert, at the beach, in the sequoia forest, etc.

I can't imagine a 3 year old really wants to take and long plane ride to visit New York City!

Besides the Portland idea, an easy road trip up to the CA central coast would be fun. Pismo Beach up through Monterey, maybe up to Santa Cruz. Lots for a young child. The Monterey aquarium is great.

I was going to suggest portland/oregon coast too....but if they live in LA, the beach isn't really "novel"....

Since you are in LA, have you considered the boat to Catalina Island for a 3 day weekend? The boat would be a novelty to a three year old and you could do the glass-bottom boat once there and also the island tram tour. Plus there is the beach to play on and build sand castles and ice cream shops right on the beach.

Sequoia was also nice. We took our oldest there before he could even walk - although he did have lots of dirty-kneed pants and dirt in his hair, mouth, all over! But he loved the nature, the openness of it all, the wildlife. You can't package that.

Our sons also liked Yosemite because we brought our bikes with childseats and rode around the valley when they were little. That got us to areas that would be too far for them to walk. They played beside the river and we were all particularly excited when a whole group of deer crossed the stream and walked right next to us. They even climbed to the top of Vernal falls at ages 5 and 3, although we did carry the 3 year old most of the way piggy-back style I admit.

They didn't have those nifty little leashes for toddlers that I've seen around so much when my boys were little. I don't like the way they look (if seems wierd to put your kid on a leash like he is a pet), but if I had a 3 year old and was going to do the national park stuff again (like we did before), I would for sure get one just for the reason that the previous poster mentioned - no one falling into boiling mud pots.

But on vacation with a child of that age, you need to be on guard whereever you go. They can move so fast and dart into a busy street or slip away before you even realize it in a crowded amusement park. So have fun wherever you decide to go and be safe.

Silly1-

I completely agree! I love St. Louis (weather aside) - grew up there, still have family there, and visit often (Go Cards!) I just kind of see it as a hard sell as far as it being a destination for someone who doesn't have another reason to go there.

Three year olds do remember a lot, so do pre-teens, and I remember being most excited about the hotel pools on our big vacations to San Diego Zoo, Disney Land, Grand Canyon, etc. when I was growing up!