Take a large plastic bag, and about 20-30 balloons.
Partially inflate the balloons, to about half their maximal size (or better yet - use the balloons left after a birthday party!).
Put the balloons in the bag and seal (with duct tape).
You now have a real pouf, which you can sit on, lie on, roll on... a real treat!
How come the balloons don't pop?
You can conduct several experiments to understand how this happens: blow the balloons fully, so that they're very tense (it is well to prepare for the noise, and place the pouf on a soft surface); blow them just a little, and see what happens; blow different balloons to different degrees, and observe the consequences. Inflating the balloons to a medium size allows the balloon to be soft and cozy enough, and yet loose - thus strong - enough as not to pop. The child's weight is split between all the balloons, thus each one carries just a relatively small weight.
Highly recommended! :-)