Pillow fights are probably the most stereotypic slumber party activity. I mean who hasn't seen a movie with a slumber party pillow fight? That's what I'm saying:)
Well, it should be self explanatory, but in case it's not... basically it's hitting each other with a pillow. It's fun and nobody gets hurt.
Spontaneous pillow fights are the most fun. Just grab a pillow when someone says something silly and hit him/her over the had with it. When all hell breaks loose, just swing that pillow like crazy:)
You can also plan a pillow fight. Get set up in teams and try to get the other team to surrender or until one team is "disarmed". Attack!