Baby showers!

While the game lacks this as a named option, gameplay-wise you could still kind of do this in a couple of ways. While this is not an update or added content, necessarily, the game still bends to "new" kinds of ways to play when you get innovative. Please note none of these mean the game would not benefit from a baby shower event, these are just ideas to spice up your game.

If you want to emphasize the social even aspect, you could use something like a dinner party to invite friends over, decorate the house or gathering lot in festive pastels and serve food and drinks suitable for your expecting sim! While the game does not recognize this as a baby shower in the name, it would get you really cute screenshots and vibes still.

If you want the game to recognize the baby shower in name, you could also use the Seasons calendar to plan a calendar holiday with custom activities and a decoration style, and add activities like "invite friends over" to the goal if you like. This way your sims could experience satisfaction (and satisfaction points) from the event. Remember to use the seasonal decorations box to set up some additional cuteness!

Third option is to invite the friends you'd like to come along somewhere. If you have Dine Out or Spa Day, a restaurant visit or a spa treatment with friends might be a great way to celebrate a baby shower, or if not, why not a museum trip. Or perhaps you could do something more adventurous like spend the baby shower in a national park, hiking Mount Komorebi, or camping!

This is just my observation, but I have found the gameplay becomes more meaningful when you pay attention to details like hand-picking holiday decorations for events, the children of the family making festive crafts for a party, planning a cute, holiday appropriate foods to serve, and downloading special lots from the gallery to celebrate.