At school I had a water pistol (squirt gun in USA?), and one day furtively filled it with dilute sulphuric acid from the chemistry lab.  The fun we had!

NB This wasn't intended for squirting fellow schoolmates (though there were one or two that I wouldn't have minded hosing with it).  It just made an impressive sizzle when you squirted rocks.  Before the day was out the plastic melted and rotted my pants pocket -- it all got a bit Alienish.

Stolen railroad detonators made a pretty good bang if you hurled half a brick at one.