Healthy small snack

Try mini snack sized cucumbers. Delicious and healthy. If you didn't want one by itself you could have it with some hummus (also easily portable if you put it in a small container!). Carrots and pretty much any veggie you enjoy raw would work as a snack as long as you have cut it up previously and have it ready in the fridge to eat or pack up to take with you.

Popcorn is a great one as well. You can air pop it in a machine or a paper bag in the microwave. Or if you feel you have a few extra calories available then pop them in a pot with a small amount of oil. You can have heaps of the stuff for minimal calories. You can also change the flavours around by adding different herbs and spices, or even parmesan if that's your kind of thing. Super portable in a container or plastic baggie, and really easy to control your portions with as you get heaps for so few cals.

Rice cakes, not necessarily "healthy" but not unhealthy. Great with peanut butter,hummus,cheese,salsa, etc.
Fruit! Apples,kiwis,clementines, grapes,etc

Nuts, jerky, cut-up veggies, crackers, snack-sized chip baggies, peanut butter, homemade muffins and breakfast cookies, kale chips, pre-made protein shakes, snack/protein bars, hard fruits like apples and oranges...

Pretty much anything that you can throw into a bag and feel confident that it isn't going to be crushed.

Luna bars, triscuits and low fat cheese, fruit, granola bars (not the Quaker oats ones! They have too much sugar)

Yoplait Greek 100 yogurt.