You know what we always used to do?

Put a mattress in the swimming pool, usually takes a crane to get it out.

The best sleepover idea is the following :
-get a bouncing ball
-get as many moto helmets as there are players
-lock yourselves in a little room
-optionally, secure the windows
-throw the ball like the silly fools you are

when i goto a slumber party
we normally attack the shit out of each other with massive ass pillows
i have one that has feathers in it and a bit of foam inside so
the spikes of the feathers and the hardness of the foam is a killer
i dont get invited to partys anymore

Where I live we have very heavy mattresses... Pillows are pretty heavy too.
But any pillow fight at my house got pretty violent, like people getting mad, using their pillow as a boxing glove and punching you... Or coming up behind you, wrapping it around your face and pulling you down to the ground so you landed on your head.

On a school trip I went to we stayed at a hotel where the pillow were a big long cylinder shape, those were great for pillow fights.

Long story short, pillow fights can be pretty brutal.

my friend would stuff his pillow with a telephone book. needless to say the pillow fights stopped very soon.

I actually had a mattress fight.. It was at a hotel and they were single beds so the mattress weren't that heavy since it was just a decent hotel. But jesus christ, I got slammed by the corner of one of those things and thought Mike Tyson had come back from the Grave.

First thing that came to mind was a fun game on Mission Trip called "Human Jenga". You stack air mattresses in a formation similar to a jenga tower, and have someone on the top air mattress. Then people take turns charging the tower and trying to knock out a mattress piece, just like jenga.

Come to think of it, I'm not surprised it was banned soon after. :Y