It's a long time ago for us now as dd is now a young adult, but we did long haul from her being 2 onwards.
We just had a normal plane seat - lots of room for a young child in them.
For occupying them we took them their own hand luggage and filled it with quiet, compact toys and treats, which we brought out at different times throughout the journey, spaced out.
Things like books/magazines (such as the CBeebies ones with plastic tat in the front that she didn't normally get), mini aquadoodles mat, sticker books, small travel Playmobil set, colouring, etc. We also packed children's over ear headphones (and practised using them beforehand) - dd was fascinated with the seat back screens and would watch the child's programmes with headphones or on silent. These days you could also pack an iPad too - didn't have them when dd was small but I'd have definitely been happy to bend screen time rules when in a long haul plane.
Little snacks (and drinks bought after security) to be brought out every so often as she might not like the plane food, though may be fascinated by the plane food containers.
Also pack a small bin liner - for the snack rubbish but also for the meal rubbish. Children are often fed first - so can end up with their tray in front of them for much longer, which can be restrictive and a source of frustration for them. If you have a small bin liner/carrier bag you can get rid of all the rubbish under their feet sooner. I do that for myself on long haul as, as a non meat eater, I also often get my food ages beforehand.
You may find she will also nap.
And don't be worried about talking her for a little walk up and down the aisles if she gets restless - other passengers are less likely to mind that than a restless child standing in chairs or knocking their seat backs!
In the airport - find the children's play area for a bit and give her chance to run around before hand. Basic,ally try to tire her out as much as possible.
Has she been on a plane before?
If not - get a book about it or watch some CBeebies type programmes about going on a plane and in an airport, so it's familiar to her. You could also role playing being in an airport/plane with her and her teddies at home.