Take things that don't have parts to fall on the floor or get in between the seats. I have learned that traveling with my 7 year old.
When mine was your little ones age, I would give him a small dose of Tylenol just before the plane began to taxi out of the gate. By take off he was relaxed and a lot of times falling asleep.
Mine was/is a very active boy. But, he tends to sit still and behave on planes. Allow him to move around as much as possible in the airport. If she is potty trained, take her to the bathroom just before boarding time. Don't let her know there is a bathroom on the plane, or she will HAVE to go. Take a deep breath and don't let yourself get flustered, they can sense it. Be sure to carry a carry-on that will fit under the seat, not one that has to be put in the overhead. You will want to be able to reach it frequently, so put it on the floor in front of you.