Glazer’s idea looks good to me.
Fill the balloons with confetti, and when she opens the bok, use a …wait…pin to pop a few of them. That way the energy that pops them makes them fly everywhere, instead of just dumping them on the box she is opening.
HOw many SDMB-ers does it take to screw in a … ahh never mind. 
Have a ‘bed of nails’ made of pins on the ceiling above her. Have the balloons not on strings.
I did this at a much smaller scale with a bottle of champagne in a box. I put the bottle in the box and then stuffed cut paper streamers around the bottle and then confetti on top of the streamers. When they pulled the bottle out the streamers dragged all the confetti out with them.
With the balloons it would take a whole bunch of streamers, but if you packed them in the box tight enough you could pack the streamers on top to hold the confetti. A lot of it might fall back in to the box, but it’ll make one hell of a mess anyway.
Or you could tie the streamers to the balloons and kind of roll the confetti up in them so when they went aloft they’d unroll and release the confetti. The seven hands thing would help here.
So how did you pull it off?
And have the extra five arms come in handy for other things?