My son's is games. Tabletop, board games, computer games, phone apps, etc. He's made his career in the field now.
He also likes computer programming, but in many ways that is also a vehicle to implement his first interest.
He also has been drawn to learn little bits of multiple languages. I think there are 4 now he has various levels of working familiarity with. Learning them seems to be strictly for fun; he much prefers learning from phone apps google resources than taking a class. How language changes and evolves is part of that interest. Again, I see connections to the game and programming interests: they are all mechanisms of expression, communication, story-telling, and connection within a set of rules and structure.
_________________
Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).