My favorite is Kids Room Stuff. The puppet theater is a great way to build Social skill, especially if you've got toddlers in the house because the kids can entertain the toddlers with puppet shows. I play on Long lifespan so I have my kids start with the base game furniture, then they "graduate" to either the Kids Room or Parenthood furniture after they finish their first aspiration.
Close second is Bowling Night since Seasons added bad weather -- bowling makes a good activity to get Sims out of the house if it's rainy or snowy. I converted The Old Salt House in San Myshuno's Spice District to a lounge/bowling alley and it's a great place for my Sims to duck into if there's a festival and the weather is miserable.