Here are our strategies for our 5 yo and 3 yo travelling transatlantic.
1. Try to get the bulkhead seat....lots more room and they can play at your feet when the seatbelt signs are off. This is not always possible, as sometimes there is a bassinet there and they will only assign it to passengers travelling with infants
2. Stock up on a bunch of cheap, small toys. Unpack from their boxes and repack in ziploc baggies. You will be surprised how little space they will take up once they're out of the packaging. Hide them from the kids and produce one every hour or so. I find little figurine sets like playmobil, or disney character playsets great. Lego also make some small sets (Harry Potter etc). The downside is that these sets usually consist of a ton of little small pieces so you'll be spending some time crouched down picking up the bits from under the seat in front
3. Playdoh have some great little small tubs with tools on the end. Buy one or two of these, spread a newspaper under the seat and let her enjoy. Downside is that you may end up with playdoh on your clothes
4. Print a bunch of coloring pages off your kid's favourite sites. A google search usually yields a ton of stuff and unlike coloring books, you can bin them with trash before you get off the plane. Sticker and decal books pack light and will also burn up 20 mins or so. Try to find some novelty crayons (we bought a great pack of crayons shaped as Disney characters last year) to interest her.
5. Buy a small notebook of colored paper, a glue stick, some glitter pens, some confetti or paper shapes and some fluffy stuff (check out any craft shop). OK, she'll make a bit of a mess, but she'll be happy (the FA's are really going to love you). This stuff will all pack up pretty small in some baggies and can also be left in the trash. I have yet to meet a kid who doesn't love sticking stuff onto paper.
5. I am not averse to giving a dose of phenergan once on board. I'm not an advocate of drugging your kid, but having spent a short (3.5 hrs but it didn't feel short, I can tell you) flight listening to my 2yo wailing for her stroller interspersed with a 20 min nap, I think a dispensation is in order (for your and your fellow passengers sanity
) If you are going to take this tack, make sure and try a test dose sometime before you go, as some kids will have a paradoxical reaction.
6. Avoid candy, soda and chocolate the day before and the day of travelling. I don't know about your kid, but chocolate is like cocaine or speed to my kids
7. Pack plenty of kid friendly snacks like goldfish crackers, teddy grahams, raisins, cereal bars and take some milk if your kid is a milk drinker (all I have ever gotten on flights is the little UHT cartons). A hungry kid is usually a cranky kid and the inflight peanuts or pretzels may not be to her tastes.
The downside to this is that you are basically filling one carry-on with stuff for her, but it is sooo worth it. My son has a little Winnie the Pooh suitcase that he can wheel along behind him. I usually pack some of the 'surprise' stuff underneath his change of clothes, then let him choose a few toys of his own to take on the flight which I pack on top. The rest I stash in my carry-on. He's so used to getting all his little surprises that he looks forward to all our flights now, and never seems to get bored.
Hope this helps...enjoy your vacation.