Build a pillow/blanket fort.
Have a pillow fight.
Play a kids board game or card game.
Play a ball game and make up the rules as you go. A la Calvinball. See this reference and this example.)
Climb a tree and read a book in it.
Play tag and/or hide-and-go-seek.
Slide down hills on big pieces of cardboard.
Go to the park and play on on the swings and jungle gym.
Wear brightly-colored, fun clothes.