If you're friends with them on Steam you should be able to see their wish list. That's what I usually get my friends for Christmas, just something gifted that they've already flagged on their wish list.

A second vote for Core Keeper here. I think it's a safer option than Terraria, which could also be a good option but isn't for everyone.

Satisfactory was a phenomenal video game that you guys can play together. Not quite like Minecraft, but similar.

Factorio trust me

Terraria

Do they have a wish list? That's what I'd recommend doing. Most people don't play games they're bought by other people I've learned that very well over the years, but if it's on their wish list already there's a good chance they will.

Try Celeste. It's not obviously about being trans but it's a really good puzzle game that has a story they might find relatable.

"How Celeste became an icon of trans gaming" article about it

A short hike

Look for a humblebundle that donates to a cause they would care about.

Have you considered sending them a physical letter? You can put a lot of digital goods as codes there (even steam games, albeit bought in 3rd party stores), also gift cards like '$10 in X store chain' if that's a thing at their place, but the novelty (hehe) of getting a birthday card that they can keep is waaay more cool than a gift notification. See if that's possible and is up your alley.

As stated by others, their Steam wishlist is always a good starting place.

If they like story games, To The Moon is like a book you wish you could forget so you could read it again for the first time. It's best played with 0 spoilers.

If they are the type of person who tries to optimise everything and automates a lot on Minecraft, Factorio (a bit expensive) might take over their entire life for a few years.

If they enjoy puzzles and/or you want to play with them, Portal 2 has a co-op section.

I will happily agree with everyone who mentioned Stardew Valley. It can be single or multiplayer, and has a large number of mods. The Expanded and Ridgeside mods add a huge amount of content including trans characters.

Depending on how well you know them, you could try asking if there's any game they'd like to play together that they don't have. Having a friend to play with is often better than the game itself.
PULSAR: Lost Colony and Void Crew are mediocre games made fun by forcing interaction with other players (friends or random people in open games). They also have a surprising proportion of LGBTQ+ players.

Noita (<- Minecraft)

A short Hike (<- Webfishing, Stardew Valley)

And just a suggestion: since you mentioned that they had a switch: Maybe try finding a used copy of Animal Crossing : New Horizons and bring it at that convention (this would be an amzing suprise but pretty costly depending on how you intend to do this 13h trip)

Dave the Diver

Terraria.

13 hours is not that far if you're in the states... You should go and meet them!!

Just kidding, good luck in whatever you decide to do; your heart is in the right place!

RimWorld or Terraria maybe?

No Mans Sky might be a good idea.

If you have a venmo or other money-transfer method, consider cash.

While a thoughtful gift is nice, cash is flexible. Enough small gifts, and one can flee home...

Vintage story.

https://www.vintagestory.at/

If they like story games Oneshot is a good idea. You could also send them Steam credit so they can pick one for themselves, like sending them a gift card.

Any idea if they are playing the games solo (where possible) or with a group? They do all support multiplayer, and it's kind of important to understand why they play the games they play.

I'd probably check the wishlist or literally ask.

Do they like coffee, tea, good chocolate?

Since they like Stardew, I would recommend Coral Island! It's like Stardew but Mermaid themed. And I believe they're working on a co-op mode for it too!

Idk, but if you want some cool indie game recs: Pseudoregalia, Dread Delusion, Demon Turf.

Core keeper.

It's a co-op base building game similar to minecraft only top-down pixel art graphics. Easy to sink hundreds of hours into, and if you get yourself a copy as well, then you can both play together

Definitely Stardew. Everyone should have Stardew.

They're trans. Is it safe to assume New Vegas is in their library?

Hello fellow kids! Which of these promoted titles should you buy?!

atlyss!