It depends on your child, I think. I have heard of kids who need a lot of their own stuff to be able to handle a trip - my impression is that these are the highly regimented kids on strict schedules who are not used to change. Or maybe just really sensitive kids?
My 3 yr. old daughter does OK with her own clothes and a couple of toys, plus crayons, etc. for restaurants so she doesn't get bored. We are taking a 4 hr. flight in a week and this time we let her pick out a new coloring book and some stickers which we will wrap and let her open on the plane. We will also bring a Walkman and a couple of her books on tape or kids' music tapes to listen to. Grandparents will be a great diversion!
Early on we thought she would have a tough time on planes (because that is what you always hear complaints about), but she almost always falls asleep for part of the time and does fine otherwise. You might want to bring plane snacks and extra water, because the meals are pretty skimpy to non-existent these days. When we stay in hotels, she sleeps with us because we prefer not to use the hotel cribs. I'm not sure they check them, set them up, and clean them properly, but maybe I'm being paranoid.