Two ways to look at it, something useful for her, or something she can give to her relatives.
I think you are right that straight cash in this situation is possibly innappropriate, although cash would be the most useful thing for her probably. If you give a gift card to Target with a card stating you want her to buy something special for her trip, then she knows your intent is to help her with the trip, and if she doesn’t really need anything she can just buy groceries with it. Assuming you have Target superstores where you live or something similar. I like gift cards to that type of store, because if the person needs nothing, they can always buy food. I hate having a gift card sit in my wallet for months.
If there is some way to find out what types of things they bring to their family from America (I assume you live in America) then buying more of that or a gift card to the store where those things can be bought would be helpful.
I like candy too, just be sure it is allowed to travel into her country if you buy it. Usually packaged food like candy isn’t a problem. My girlfriend brings back candies from Russia, I have brought candies from many places. But, what will matter is the rules for entry in her country.
In my experience there are very specific requests when people come back home from the US. My relatives rarely brought trinkets from America, but big time shopping items that they could not easily find in their home country. Or, they brought things they could sell. I don’t say this to discourage you from giving something not on the list though.
If it is difficult to come up with “American” things to send down with her, you could pick something that represents your city or state.
Remember if it is for her travels make it small and light. The airlines are charging like crazy for extra luggage and extra weight in luggage, and she might already be bringing a lot with her.
Is it a long flight? If so, you could maybe buy something that pampers on the flight. Blow up neck pillow, eye mask, Evian face spray, chapstick, travel toothbrush (there are little throw aways with toothpaste already on them) etc. You can put it in a gift bag, or if you want to spend more you can give it in a make-up/toiletry/travel bag. Although, international flights tend to have a lot of things already like pillows, blankets, and eye cover. Even if they do provide a lot of it, it is thoughtful I think. Many of those things can be used anyway, even if she doesn’t use it on the flight. I like practical gifts personally.