Like most every other free-to-play game, players buy cosmetic items to spruce up their characters, and they do this with a “premium currency.” Think of a premium currency like a casino token: it’s similar to a US$, but can only be used in a single establishment. In the case of Fortnite, the premium currency is called “V-bucks”.
So: you can’t go wrong giving V-bucks. Giving V-bucks, however, is a little complicated.
There’s no way to buy V-bucks directly, and there’s no way to buy a physical object you could put into a stocking or something. If you want to “give V-bucks”, you have to give a gift card for the platform that your giftee is playing on. So if they play on Xbox One, give them a gift card for Xbox One. If they play on Switch, give them an eShop gift card, etc.
There are two exceptions here: PC and Android. Both of these platforms use the Epic launcher and players on these platforms make purchases directly from Epic. In that case, you’re more or less out of luck when it comes to a “here’s a gift card for V-bucks” sort of item. Your best bet is to just use a regular old Visa gift card. If you’re giftee plays on Android, do not give them a Google Play gift card, because free vbucks no human verification does not run through Google Play.
So that’s V-bucks. V-bucks are the most versatile thing you can give your Fortnite-er. If you want something to wrap up, your best bet is the Deep Freeze Bundle, which comes with a couple of cosmetics along with some V-Bucks and allows you to give a physical object. You should still be able to find this on store shelves: essentially the entire point of this thing is so that Epic could put a physical object on shelves for stumped gifters to ring up at the register. Feel free to indulge.