My parents kindly let me have a party at home for my 18th, it was also about 30 people. They were good friends with our neighbours so they went over to theirs for the evening so they were close by out of the way, and came back at midnight which was the “end” time for those not staying over (about 5). I think there was pizza. Some things I remember happening:
One girl who wasn’t a super close friend so wasn’t staying over threw up all over the downstairs toilet, she was in such a state that one of the designated drivers who was supposed to be taking her home refused and left without her (don’t really blame them!) So when my parents got home they had to try and get hold of this girl’s mum to come and collect her.
The downstairs toilet also had a door that stuck a bit, someone (never found out exactly who) ripped it off its hinges.
As others have said, the floors (mostly tiled) got absolutely filthy with people going in and out, I had to help my parents wash down and repaint all the skirting boards as they got so dirty.
Back in the day a lot of people smoked so cigarette butts all over the garden, but I doubt that’s a problem these days!
Luckily my parents were very chill and I think they had mentally prepared themselves for some damage! I remember another friend’s parents who absolutely flipped when their carpets were left filthy after a party at her house and she had to sheepishly ask us all to contribute to the cost of a professional carpet clean that her parents had insisted on.