It is even difficult for some people, like myself, to come up with a wishlist for birthdays or Christmas, especially when people ask you what you want and don't take "nothing" as an answer.
This is a pretty broad category, but there are so many travel accessories that are great to have that people don't always want to spend their money on.
Some generic ideas of travel accessories include luggage tags, bags for travel (duffel bag, suitcase, backpack/Camelback, reusable compact bags, laundry bags, toiletry bag, fanny pack, etc.), passport holder, reusable water bottle, portable charger (some are even solar powered), books (particularly ones related to their upcoming travel destinations), and an outlet converter/adapter.
Not everyone likes to buy others clothing, but if you know your friend or family member is going on a trip it can be nice to get them something that they may not have and don't necessarily want to spend their own money on!