Days off…

Whatever I feel like they want to do for that day. Not every minute of every day in a sim's life needs to be doing things for a purpose other than "because they feel like it". What, specifically, I'll have them do all depends on the sim(s), their traits, and their lifestyles, and runs the gamut of everything the game has available to have sims do.
For households with children, if the children also have the day off I might make it a family fun day if it's been a while since they've done that.

Ultimately though, I just wing it. No 'routines'...routines are boring.

I try to think about what I probably would be doing. They catch up on cleaning or aspirations, maybe hang out in the backyard, swimming and playing. Sometimes they'll go picnic and fishing or camping for the weekend. They may have a house party, or keg party. My teen sims may hang out in their room playing video games or spend all day on the phone working up their friendships. If it's my rotational save, my sims will visit their family members and work on their relationships with their siblings, parents, cousins, or nieces/nephews, etc.

My sims vacation, go to the spa, visit family and friends, have dinner parties, go out for meals, bowling, skating, scout meetings, movie nights, etc.

Usually I take the opportunity for my sims to have fun at the weekend if they've worked or been at school all week.

Visits to the beach, the park (with kids), the cinema, bowling, dining out, a day at the waterworld, getting married, parties, vacations to other worlds, going to a bar for karaoke/foosball/darts/cards, visiting friends etc.

I do have a few more studious sims in my rotation who like to visit the library lol.

I usually take cue from the whims and do something they wanted to do like go skating or use the time to fill up their fun and social bars. A couple of sims are into gardening and they go to the Finchwick fair. Some sims work on a hobby like mountain climbing or Batuu. It depends on the sims, their interests and hobbies, but they usually go out a lot.

Movie nights with ice cream and popcorn. Going to the beach. Karaoke. Dance clubs. Hosting dinner parties. A short trip to the jungle or granite falls. Scout activities for kids. Soccer games on Friday or Saturday night. Pubs. Etc

I usually have the kids catch up on homework, and other school requirements first.
After that, all my Sims visit other areas, like other worlds, or go to the malls.

My Sim who just became a YA used time off to do concerts, photoshoots, bit parts in movies, and lay down tracks for an album. Or even to finish writing another song. Right now, my Sims are struggling with the stomach flu, allergies and various other ailments. So, some of them spend time at the Clinic to be diagnosed. That can take up some of the time and is generally interesting.

My teens to Batuu (as an amusement park).
Other times they go to Mt. Komorebi to switch things up.