Very lighthearted one but I'd say clothes shopping. Stereotypical reason but clothes shopping is fun and a stress reliever for a lot of girls, because we have way more fashion choices and styles and choices and all that over men. I think some guys get that but it can be hard to appreciate because most guys (def not all) seem to not enjoy it anywhere near as much.
The big thing I don't think men get is how hard shopping for women's clothing can be. Walking into a store is like walking into a random apartment during a zombie breakout looking for supplies. You have no idea what you're going to find, it could be amazing stuff, it could be a big bust. I've been clothes shopping with guys and it can break down into very simple ideas, like "I need a red shirt" or "I need a nice pair of jeans". And you can walk into almost anywhere, and find a red shirt, or a nice pair of jeans.
I wish all men had to dress in women's clothing for a month and go shopping for that clothing to really understand how it works on our side. There is no "red shirt" for the most part, because this top may be cut one way, or that one might have extra decoration on the sleeves, or whatever. Even a basic t-shirt isn't always basic. The nice part is that there's so much diversity and choice but that then means one singular item can be created in hundreds of different ways and colors. Clothes shopping as a girl is less about going out specifically looking for something but instead hoping you find something good, and doing so can take hours sometimes depending on how you luck out. And then when you find something cute, every brand's sizing can be different, and the way clothes fit can vary so widely that it can be soul crushing when something you were sure was going to look good ends up looking terrible on you.
Plus if you've got a girlfriend/wife/whatever that likes to look good, remember that she's doing it in part for your sake!