by Lawyerish
I recently traveled with my 20-month old daughter from New York to Florida by myself (my husband had a work conflict), and our trip included a violent bout of carsickness on her part as well as a lengthy flight delay, in addition to the 2.5-hour flight itself. During this test of strength and character, I mentally kept track of things I was really glad I'd brought with me, and things I desperately wished I'd thought to bring in my carry-on.
Let my experience be your guide, and pack the following in your carry-on:
1. Lots of plastic bags, extra clothes for the child and for you, and ten times as many baby wipes as you think you need. You already know to pack fifty diapers for even a short trip, because WHAT IF you end up stranded somewhere or WHAT IF your child gets explosive, ahem, intestinal issues during the flight, right? But you might not think to pack extra baby wipes. (I didn't, anyway.) Wipes will come in handy for so many contingencies (spills! vomit! stickiness!).
After mopping unmentionable things off my daughter's carseat with half a pack of wipes, I started to sweat when I realized how few I had left. This was not a good feeling. Also, after her first outfit was destroyed in the Car Sickness Episode, I was fortunate that the cab driver had given me a plastic bag, but I could have used more during the trip. Next time, I'll shove a bunch of grocery bags in my carry-on, as well as big Ziplocs, to contain any Yuck that happens. It's also handy to be able to pack up your copious trash and hand it to a flight attendant as they prepare for landing. And I will be packing an extra shirt for myself in addition to TWO extra outfits for my child, because you can just never be too cautious about these things, and again I was worried the whole rest of the trip that she'd already blown through one outfit and I wouldn't have more until we landed and retrieved our suitcase.

2. Loads of non-messy snacks. It's really hard to entertain a small child who isn't yet capable of engaging with stickers or crayons, but who might not nap in-flight. Eating, however, is entertainment for them, especially if you bring something fun and time-consuming for them to eat. I'm not recommending sugary snacks, because then you have a potential for a sugar-crash problem, but I'm thinking those purees of vegetable/fruit in pouches (my daughter ADORES those), puffs in a snack trap, Cheerios, cereal bars, Toddler Mum-Mums, string cheese, small crackers, teething biscuits, cut-up pepperoni -- WHATEVER IT TAKES to keep them quiet but doesn't get them over-stuffed.
Side note: I do not recommend yogurt, as it can easily get all over everything, and no one wants a yogurt-smelling kid next to them in-flight (as I now know too well). And don't forget a sippy cup (preferably with a straw for even less spill potential) filled with water!
Another side note: I put all of my snacks into quart-size Ziplocs that were easy to pull out at security and run through the machine, and the sippy got hand-checked and tested by TSA.
3. Infant pain reliever. Helps with their popping ears and MAY relax your child enough to sleep a bit on the plane.

4. A backpack leash. I KNOW. Judge if you must, but I was SO glad I'd borrowed one of these from a friend. During the flight delay, my kid was all over the place, running amok in a very crowded area, and this helped me wrangle her without having to coop her up. She actually loves wearing the monkey (she puts it on at home sometimes), PLUS an older girl came over and wanted to hold the leash and the two of them had great fun playing. I got a lot of comments and bemused/horrified looks from bystanders, but I couldn't have cared less. My child was CONTAINED without being cranky about it. WIN.
5. A bunch of (preferably new-to-the-child) books. I brought about six books on our flight since reading and singing songs was about all I could do to keep my child engaged while she was strapped into her seat for hours on end. Luckily, she has a long attention span for books, so I could read the same ones (several were new, which added to the enthrallment) over and over. Books that have flaps to lift or things to touch are also a bonus, though they can be bulkier to pack.
As a bonus tip: If your child has his own seat (OMG, spring for the extra seat, PLEASE -- if I had had to hold my squirmy toddler on my lap I would have leapt out the emergency exit shortly after take-off), have him sit in his carseat (I rolled my Britax Roundabout through the airport on this, which I reviewed here a while back) and install it in the airplane seat rear-facing. I made the mistake of having my kid front-facing on the way down to Florida, and she kicked and pushed her feet against the seat in front of her THE WHOLE TIME. I could not get her to stop and it was very embarrassing. On the way home, I put her rear-facing and she was much more comfortable (her feet didn't have to dangle) AND couldn't bother anyone else. Quel relief.
Good luck and safe travels!
(Photos: amazon.com)