Katie, my favorite go-to snacks are fruits and veg - just whatever we had in our CSA basket this week. And as for portable, apples and citrus transport pretty well, and soft fruits can be washed and bagged or put in a tupperware. Cucumber slices and carrots carry well in baggies. I pretty much only cook for meals. Snacks are more likely to be a wash-and-chop affair!
I also really like almonds and sunflower seeds, and the kids dig Trader Joe's dried bananas and dried mango and sesame seed crepes (Katie P introduced me to those!), though the last does have a fair amount of sugar and is processed - but very portable and has a fair amount of protein. Oh! And hard-boiled eggs! The kids like them and I do too. If you boil them with salt in the water they're great right out of the shell.
And - not portable - my favorite snacky snack is popcorn. I air pop it and add melted butter, salt (you can get popcorn salt that's finely ground so it sticks better) and grated Parmesan cheese. Soooo good. And not high calorie if you go light on the butter.
Hope it helps, and I'm looking forward to read the others' suggestions!