We've always used Goldfish, Annie's Naturals Graham Bunnies (they come in individual packets), mini rice cakes, and tons of dried fruit, fruit leather, CLIF has a new kid's fruit twist that actually counts as a serving of fruit and is all natural -and if constipation is an issue -FREEZE DRIED FRUIT!!! This is different than regular dried fruit. My son's favorite is found at Whole Foods in the section with the bins of snacks and dried fruits. It's called Not Just Peas or something like that and comes in tubs. You can also find travel pouches of it at REI. Be warned -it's expensive! Other brands I've found are also expensive, but some come in smaller packets and don't cost so much at one time. They retain all of the nutritive benefits of fresh fruit, and be prepared -it will clean your child out!
We use the natural, dye-free, Mott's gummy packets as treats and on the go treats. They really taste yummy and have a full day's value of Vitamin C in them. I've also yet to find a child who doesn't love Pirate Booty -especially the original cheese flavor. They've started selling this in individual packets for on-the-go in the health food section of Kroger and Whole Foods as well. Trader Joe's is another wealth of snack finds. None of their stuff has artificial dyes in it, and a great deal of it is organic if that's important to you.