Trip

One of the best things I bought for my kids was a box of those triangular-shaped crayons. They don't roll all over the place, which was very helpful - you can get them at Target.

I second the recommendation for a small DVD player or laptop. That was the number one thing.

For the plane ride I would just take lots of snacks, new books, small toys and a portable dvd player. I have traveled a lot with my kiddos and I have found that new things always keep their attention better. I also always carry a little bag of candy (things they usually don't get to eat at home) like marshmallows, suckers, chocolate etc. So if they do start to have a melt down on the plane I can whip that out and they calm right down. LOL

Take any special item like a blankie or a special stuffed animal for security, then everything else should be new and exciting! Especially for the plane ride. You need something for the child to drink during take-off & landing to prevent the ears from popping. You'll have to buy it once you get past security since you cannot take liquids through. Otherwise get something from the flight attendant as soon as you board, since they don't break out the drink cart until well after take-off. Keep your child (and yourself) hydrated during plane travel - that recycled air can be a pain.

Have toys that don't make a lot of noise, for the sake of other passengers. If you use a portable DVD player or something like that, see if you can find kiddie headphones. That can be helpful since the plane engines are noisy too! Books with pop-up pages or touchable surfaces can be interesting.

Lots of little snacks are good. One post mentioned marshmallows - if you do that, use the tiny ones. Kids choke on them. Also, not too many in the mouth at once - they melt and glop together to block the throat! (And that goes for older kids too - they have contests to see who can stuff the most marshmallows in their mouths, and then someone winds up with a blocked windpipe. Scary!) Anyway, I agree that giving her a few treats she hasn't had before is a good idea. And take her favorites too of course, including protein. PB&J sandwiches are good and they keep for a while without refrigeration.

While in the airport, walk around - you should take an umbrella stroller. (flight attendants will check it for you later). Show your child all the sights, all the people, all the planes, anything. It will make it easier to sit still in the plane. Take advantage of pre-boarding for children even though it means more time on the plane. You'll get better seating and have good access to overhead storage. Sit near the front of the plane if you can - you'll get off much faster.

Beforehand, rent a video about plane travel if you can, or get a book from the library. The more familiar your child is with what will happen, the more fun it will be.

Check as much baggage as you can and just carry on what you will need for the flight. Consider a small pillow and blanket too. Some people take the carseat on - it's safer of course and it's a familiar thing for your child to be strapped into with no expectation of getting out. Hopefully you will have someone else with you to help you tote this stuff around!

Enjoy!

Here are a few lists of things to bring.
http://www.bigbluetrunk.com/ourcorner/550
http://www.bigbluetrunk.com/ourcorner/341

The most important thing you can do is keep to your child's normal schedule as much as you possibly can. Avoid letting your child becoming very tired or very hungry. One thing that was a lifesaver when I was travelling with my young children was an umbrella stroller with a reclining seat. At the time my children were little MacLaren made a great one, but there may be other less expensive options out there now.