If you recall, previously I gave tips on how to be trendy while traveling. To be honest, as comfortable as you may want to be during a trip, especially one that involves lots of walking, I suggest that you treat a holiday like you would a casual party. In other words, at all times, you should dress well. I actually think that vacations are a great way to dress up.
Then comes about the wardrobe for the actual travel. I can understand why you may want to be comfortable on long flights if you are traveling in coach, but trust me, I do not want to see you in your nightwear in a plane. That means definitely no pajama jeans or tracksuits or anything that should not be worn outside the house.
And if you need some style ideas, take a look at Dita Von Teese at the Los Angeles Airport (LAX) and she just checked at the airline counter. She is basically all set to walk into any formal event. She can do a touch up just before landing if she is being greeted at the airport. If not, she can use the restrooms.