We do long flights with our daughter way too frequently - about 8 overseas trips (and uncountable domestic ones) before she was 5, and from Australia, any o’seas trip is a long one! I think the one to the Bahamas took about 22 hours all up, what with the connections and all, and it was worse on the way back. (About 28 hours. We try not to mention it anymore!)
We’ve never sedated MameBabe, but that is just our personal antipathy towards unnecessary medication. We keep her awake till the flight takes off then let her sleep when the plane darkens.
Take infant paracetamol and a measuring device, plus regular medications, her thermometer, and whatever baby health book you use, onto the plane with you. This is the voice of bitter experience. MameBabe spiked a 40C fever just out of New Zealand one trip, and there was no infant panadol on the plane.
A syringe without the needle is really good for liquid medicine, 'cos you can’t spill the stuff once you’ve dispensed it, but it needs to fit down the neck of the bottle (you think that sounds obvious? Ha!).
We take healthyish munchies for the flight, not lollies. No point in adding a sugar high to her already high level of activity.
Like everyone else, I strongly recommend a sippy cup with water in it even if you are still breast feeding, to help pop her ears for takeoff and landing. Some extra drinks (juice and/or water) are well worth it too, particularly if the plane is delayed or you are stranded on the tarmac for a few hours. Or both! (I loathe United.)
For toys, others have covered the old/new bit already, but let me add a couple of things. First is a glove puppet - walk it along the back of the seat, up her arm, hide, make silly voices, help read a book, let her try it out…etc. Second forget pencils and pencil sharpener in air, take scratch paper and retractable crayons (less likely to break in little hands) and/or a write on/wipe off board and dry erase markers. Crayola make a good zip-up one and you can shove the paper and crayons in the pouch too.
I wouldn’t take PlayDoh on the plane - a pest to get off the floor, off the shirt of the person in front of you, out of the seat covers, out of her hair, or maybe yours is less messy than mine?
For books, the Fisher-Price activity books are worth a look to see if any are age appropriate for your girl. We also have a couple of game books suitable for non-readers which we used to keep just for travel. Think “Where’s Wally” for the 2 - 6 set. One is called “Can you spot the spotty dog” by John Rowe (http://www.realbooks.co.uk/rbn/rbn_5.htm and scroll down for a description), the other is a Winnie the Pooh one called (I think) “Pooh’s look and find”. I can’t check as I have just lent them to frends travelling to England with their 2 year old!
If you’re likely to do a lot of long trips, check the convenience stores at the airports for novelty items while you are waiting. Parents travelling with bored kids are their target market! Look for stuff for the next trip or the one after. I’ve found things like magic ink maze books which MameBabe adored. Never seen them anywhere else.
Finally, I suggest you let her select a particular toy to take onto the plane to cuddle and to look after. We’ve gotten MameBabe involved in the preparations since she was about 2 - picking clothes, selecting familiar books and toys to take and so on. Participating has made it easier for her to adapt.
Have fun!