K.G.
When I was in high school, I planned a lot of those just because I was bored! And while my friends initially rolled their eyes, they always got into it and had a great time.
Some ideas:
A good, old-fashioned pillow fight
Red light, Green light
Fishy fishy cross my sea
Scavenger hunt or scavenger hunt where one clue leads you to the next clue
Kid toy gift exchange
Bubble gum blowing contest
Play-doh sculpting contest
Kid board games: Candyland, Hungry Hungry Hippo, Battleship
Have fun! And can I come?