Various things covered before, but the big one for me is spontaneity. When you have dcs you can still go away and have nights out, but everything needs to be planned.
I loved those nights when an email would ping in my inbox from DH around 5ish asking if I fancied a drink near the station before heading home, then we'd end up staying out, grabbing some food and more drinks together, nothing wild, just not the same limitations. I will never again need "day to night dress" because a night out won't just evolve.
I miss being able to spend Sundays pottering, drinking good coffee, reading the papers, maybe taking a walk without having to think about anyone else.
I miss being able to say "yes, let's go for it" to job offers in other countries without having to think about schooling and removing dcs from settled lives.
I miss being able to go stay in far flung hotels that are completely unsuitable for children, reading 5 books on a holiday, and having a body free from stretch marks in the bikini.
I miss spending rainy Saturday afternoons in bed with DH having very loud sex.
Who knows what would be on your list, you aren't me, you might be a "planner" so not want spontaneity anyway.