Sleep outside in your own back yard or balcony,
Take turns reading a classic novel aloud to the rest of the family
Have everyone, parents included, dress up in costumes and put on a show in your living room,
Have a sing-a-thon, with or without an instrument or recordings,
Take a picnic to the park,
Play games together,
Create a work of art such as a mural or sculpture,
Cook something delicious from scratch and have a feast.
Several of these things can be done on the same day and guests can be included, but to really enjoy them it's best to involve the whole family.