These are the things that i can think of right at this second. there are probably more, but i'm not going to think about it further, cause these answers are spontaneous, and spontanaitey is a new thing i'm trying.

A Jacket
You never know when the weather is going to turn...It could happen anywhere, anytime.

A Book
Long plane trips, car trips, train trips, boat trips (or any other mode of transport) can be boring. Books kill the boredom. For a while anyway. until you finish the book. and then you're screwed.

Music
Whether it be CD's, on my ipod or just in my head, music has to be with me all the time. Sometimes, (actually all the time) i co-ordinate music to what i'm doing as if my life were a movie. It's really fun.

Extra Clothes
It doesn't matter if it's only a day trip, i have to take extra clothes. I have a nasty habit of spilling things on myself and walking around a place with a bright red raspberry soda stain down the front of your shirt and pants is not a good look. In fact, if it's red enough, you could get arrested on the suspision of homicide. but that's worst case scenario.

Money
Why?? Do I even need to explain why you need money? everything costs money nowadays. Sometimes, you even have to pay to use the bathroom.

Phone
Incase i get lost, or hurt, or get bored and need someone to talk to. Or to make me feel as if someone could actually want to talk to me on the phone. Although, i'd rather see a person to speak to them. I run out of things to say on the phone, plus being on the phone feels a little uncomfortable.

A pillow
Because, sometimes, travelling makes me tired. I sometimes need to stop and sleep. Without sleep, there is no travel (does that even make sense??..no i don't think it does. But i'm going to leave it there cause it sounds a little philosophical.)