If you want to keep it shaving cream related...
Freeze a couple of cans of shaving cream, until they are as frozen as possible.
Now use whatever you can, and peel the metal of the can away completley, so you're left with a frozen solid cylinder of shaving cream.
Now if you place that cylinder anywhere, say the inside of a car, or better yet, the glove compartment, when it thaws, it will expand to about 850x its size.
Now, one can wont fill a car very well, which is why I would suggest multiple, or placing them in small areas, like the glove compartment, or toilet...
For more information:
http://www.cockeyed.com/pranks/frozen/frozen.html