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.