We live by a great beach so don't bother with sun and sand holidays abroad, we do lots of shorter holidays in the UK to Premier Inns (always the same decor, same sort of food etc so predictable), factor in travel days where that is just what we do or check in early and go out in the evening.
Know the rough plan advance, DD jokes that the itinerary needs submitting in writing 24 hours before - but there is some truth to it. She finds it much easier to remove something from an itinerary than add it on.
Noise cancelling headphones, always even if she thinks she won't need them. Try and view them like a pair of glasses which you wouldn't go out without.
Find out where quiet spaces are at venues before we go. London attractions are good for this particularly the galleries and museums.
Lots of quiet downtime, try not to ask too many questions or for her to make too many decisions if likely to be overwhelmed.
However as others have said, some sort of quirks must have been present from toddler hood onwards. DD thrived at Primary and was very bright and well liked but there were always quirks and they got a lot worse after COVID and starting Secondary school but they were always there and we have several family members who are undiagnosed but have strong autistic traits going back through all living generations.
Autism is largely genetic so if there are no signs of it within the wider family, and no signs of it before COVID then it's likely not autism.
In terms of uni, we are a few years off that yet but. A trying to work my way up the career ladder because I know sell need extra financial support, will need a single room with an ensuite although a one bed flat would be better. Will likely not be able to work and study so will need help there.
We go to events art shows/theatre productions/ museums/use the sports facilities/ at our local uni and further afield if in a topic DD likes so she can see uni life, although I would have done that without the autism anyway