i'd say you would be able to burn things with a 30mw, my 10mw from ebay pops balloons. a 30mw should be able to pop a balloon quite easily and probably melt black plastic, it really depends on what laser you get, you might get an over powered one or you might not.
i think it would be able to melt chocolate, it wouldnt put big smoking holes in it, it would just make a little track in it.