Don't know if she's still drinking from a bottle or sippy cup, but definetely have something for her to drink. Be sure not to let her drink from it until the moment you're actually taking off(unless of course she's really, really thirsty). She'll be filled up and not drink it when she needs to. If she uses a pacifier, make sure you have a couple on hand.
Portable DVD players are great. We got ours at
Walmart's for about $130 and just love it. Have gotten a ton of use out of it for long car rides. Worth the price for a hour of quiet.
Stop at the dollar or toy store and get some new toys but don't let her see them until you're on the plane and then only one at a time. I'd stop at the dollar store for mine, load up with about $20-30 worth of stuff. Each toy kept them occupied for at least 10 minutes and if we lost them, no biggie it was only from the dollar store.
Bring on a variety of her favorite snacks. I've even brought a small soft sided cooler on board (that could be her carry-on) and had yogurts, cheese, juice, etc.
Don't forget to bring a small bottle of childrens' Tylenol , or Motrin. With kids' , they always get a fever at the worst of times. My DS was 3 yrs when we went to WDW one time and even though he wasn't sick before we left, ended up with a fever on the plane (found out it was an ear infection). It came on pretty quick, and I'm glad I had some meds with me for the 3 hour flight.
Favorite blankie/stuffed animal would be good too.