I might have missed it, but what's the circumstances and terrain of the mission trip? Are you going to be out and about (woods, etc) building a home or in the ghetto repairing a soup kitchen, etc? That information makes all the difference.
I led middle school and later high school youth groups on several trips through high school and college, respectively. Be a minimalist. Less is more, as stated. Did I mention don't take much? Think Buddhism here, being weighed down by your belongings. Think of having way less stuff, including music and cell phones, etc, as a way of leaving the normal constraints and hastles of normal life behind.
Take a simple backpack with very comfy shoulder straps. A little bigger than normal won't hurt. The specific gear will change drastically based on the first question on what the trip is about, but certain basics remain.
A nalgene is great because they hold a good amount and are indestructible. They can also be strapped to your pack or belt.
Bring a simple first aid kit for minor/intermediate situations. Plan on having gear built around helping others, because you will. An amazing amount of girls (referring to both males and females) go on these things, and most whine and struggle with minor crap, as well as give up at the first sign of not-fun/hard stuff.
Bring just a couple of high energy snacks.
Bring one good flashlight, either lithium or LED.
Bring the LED headlamp.
Bring a leatherman.
Bring a solid 3"+ folder.
Bring duct tape.
Bring a good length of strong rope (not too much, used to strap things down, make a carry sling for the water bottle, helping people up a steep area, etc)
A bic lighter.
Pad of paper + sharpie + pen
Work gloves (after working concrete construction, I like the rubbery mechanix gloves, but leather will also do a good job of keeping uncallous hands in good shape, though the leather ones dont stay on the hand as well for me)
Good workboots (wolverines at walmart are good, and cheap. They survived work in concrete)
Playing cards and a book or two. Dont forget playing cards.
Batteries (any mp3 player, and to loan to cute girls)
2x poncho (also for cute girls)
2x bandanas
Don't be the guy who shows up to a mission trip with $200 in new gear. It's cool with the small stuff, but make what you have work for the most part. Also, no web gear. No web gear.
If food, shelter and work tools are covered, there's not much else you really need.